1Some FundamentalChemistry

1.10 Towards the end of the fifteenth century the technology of shipbuild-ing was sufficiently advanced in Europe to allow Columbus to sail westinto the unknown in a search for a new route to distant Cathay. Earlierthat century far to the east in Samarkand in the empire of Tamerlane,the astronomer Ulug Beg was constructing his great sextant—the massivequadrant of which we can still see to-day—to measure the period of ourterrestrial year. He succeeded in this enterprise with an error of only 58seconds, a fact which the locals will tell you with pride. Yet at that timeonly seven metals were known to man—copper, silver, gold, mercury,iron, tin and lead; though some of them had been mixed to produce alloyslike bronze (copper and tin), pewter (tin and lead) and steel (iron andcarbon).

By 1800 the number of known metals had risen to 23 and by the begin-ning of the twentieth century to 65. Now, all 70 naturally occurring metallicelements are known to science and an extra dozen or so have been createdby man from the naturally occurring radioactive elements by various pro-cesses of 'nuclear engineering'. Nevertheless metallurgy, though a modernscience, has its roots in the ancient crafts of smelting, shaping and treat-ment of metals. For several hundreds of years smiths had been hardeningsteel using heat-treatment processes established painstakingly by trial anderror, yet it is only during this century that metallurgists discovered howthe hardening process worked. Likewise during the First World War theauthor's father, then in the Royal Flying Corps, was working with fighteraeroplanes the engines of which relied on 'age-hardening' aluminiumalloys; but it was quite late in the author's life before a plausible expla-nation of age-hardening was forthcoming.

Since the days of the Great Victorians there has been an upsurge inmetallurgical research and development, based on the fundamentalsciences of physics and chemistry. To-day a vast reservoir of metallurgical

knowledge exists and the metallurgist is able to design materials to meetthe ever exacting demands of the engineer. Sometimes these demands areover optimistic and it is hoped that this book may help the engineer toappreciate the limitations, as well as the expanding range of properties, ofmodern alloys.

1.11 Whilst steel is likely to remain the most important metallurgicalmaterial available to the engineer we must not forget the wide range ofrelatively sophisticated alloys which have been developed during this cen-tury. As a result of such development an almost bewildering list of alloycompositions confronts the engineer in his search for an alloy which willbe both technically and economically suitable for his needs. Fortunatelymost of the useful alloys have been classified and rigid specifications laiddown for them by such official bodies as the British Standards Institution(BSI) and in the USA, the American Society for Testing Materials(ASTM). Now that we are in Europe' such bodies as Association Frangaisede Normalisation (AFNOR) and Deutscher Normenausschuss (DNA) alsobecome increasingly involved.

Sadly, it may be that like many of our public libraries here in the Midlands,your local library contains proportionally fewer books on technological mat-ters than it did fifty years ago, and that meagre funds have been expendedon works dealing with the private life of Gazza—or the purple passion publi-cations of Mills and Boon. Nevertheless at least one library in your regionshould contain, by national agreement, a complete set of British StandardsInstitution Specifications. In addition to their obvious use, these are a valu-able mine of information on the compositions and properties of all of ourcommercial alloys and engineering materials. A catalogue of all Specifi-cation Numbers will be available at the information desk. Hence, forearmedwith the necessary metallurgical knowledge, the engineer is able to select analloy suitable to his needs and to quote its relevant specification index whenthe time comes to convert design into reality.

Atoms, Elements and Compounds

1.20 It would be difficult to study metallurgy meaningfully without relat-ing mechanical properties to the elementary forces acting between theatoms of which a metal is composed. We shall study the structures of atomslater in the chapter but it suffices at this stage to regard these atoms as tinyspheres held close to one another by forces of attraction.

1.21 If in a substance all of these atoms are of the same type then thesubstance is a chemical element. Thus the salient property of a chemicalelement is that it cannot be split up into simpler substances whether bymechanical or chemical means. Most of the elements are chemically reac-tive, so that we find very few of them in their elemental state in the Earth'scrust—oxygen and nitrogen mixed together in the atmosphere are the mostcommon, whilst a few metals such as copper, gold and silver, also occuruncombined. Typical substances occurring naturally contain atoms of twoor more kinds.

1.22 Most of the substances we encounter are either chemical com-pounds or mixtures. The difference between the two is that a compoundis formed when there is a chemical join at the surfaces of two or moredifferent atoms, whilst in a mixture only mechanical 'entangling' occursbetween discrete particles of the two substances. For example, the pow-dered element sulphur can be mixed with iron filings and easily separatedagain by means of a magnet, but if the mixture is gently heated a vigorouschemical reaction proceeds and a compound called iron sulphide is formed.This is different in appearance from either of the parent elements and itsdecomposition into the parent elements, sulphur and iron, is now moredifficult and can be accomplished only be chemical means.

1.23 Chemical elements can be represented by a symbol which is usu-ally an abbreviation of either the English or Latin name, eg O standsfor oxygen whilst Fe stands for 'ferrum', the Latin equivalent of iron'.Ordinarily, a symbol written thus refers to a single atom of the element,whilst two atoms (constituting what in this instance we call a molecule)would be indicated so: O2.

1.24 Table 1.1 includes some of the more important elements we arelikely to encounter in a study of metallurgy. The term 'relative atomicmass', (formerly 'atomic weight'), mentioned in this table must not be con-fused with the relative density of the element. The latter value will dependupon how closely the atoms, whether small or large, are packed together.Since atoms are very small particles (the mass of the hydrogen atom is1.673 x 10~27 kg), it would be inconvenient to use such small values ineveryday chemical calculations. Consequently, since the hydrogen atomwas known to be the smallest, its relative mass was taken as unity and therelative masses of the atoms of other elements calculated as multiples ofthis. Thus relative atomic mass became

mass of one atom of the elementmass of one atom of hydrogen

Later it was found more useful to adjust the relative atomic mass of oxygen(by far the most common element) to exactly 16.0000. On this basis therelative atomic mass of hydrogen became 1.008 instead of 1.0000. Morerecently chemists and physicists have agreed to relate atomic masses tothat of the carbon isotope (C = 12.0000). (See paragraph 1.90.)

1.25 The most common metallic element in the Earth's crust is alu-minium (Table 1.2) but as a commercially usable metal it is not the cheap-est. This is because clay, the most abundant mineral containing aluminium,is very difficult—and therefore costly—to decompose chemically. There-fore our aluminium supply comes from the mineral bauxite (originallymined near the village of Les Baux, in France), which is a relatively scarceore. It will be seen from the table that apart from iron most of the usefulmetallic elements account for only a very small proportion of the Earth'scrust. Fortunately they occur in relatively concentrated deposits whichmakes their mining and extraction economically possible.

In passing it is interesting to note that in the Universe as a whole hydro-

Table 1.1

Element

Aluminium

Antimony

Argon

Arsenic

Barium

Beryllium

Bismuth

Boron

Cadmium

Calcium

Carbon

Cerium

Chlorine

Chromium

Cobalt

Copper

Dysprosium

Erbium

Symbol

Al

Sb

Ar

As

Ba

Be

Bi

B

Cd

Ca

C

Ce

Cl

Cr

Co

Cu

Dy

Er

Relativeatomicweight(C =12.0000)

26.98

121.75

39.948

74.92

137.34

9.012

208.98

10.811

112.4

40.08

12.011

140.12

35.45

51.996

58.933

63.54

162.5

167.26

RelativeDensity(SpecificGravity)

2.7

6.6

1.78 x10~3

5.7

3.5

1.8

9.8

2.3

8.6

1.5

2.2

6.9

3.2 x10-3

7.1

8.9

8.9

8.5

9.1

MeltingpointCC)

659.7

630.5

-189.4

814

850

1285

271.3

2300

320.9

845

640

-103

1890

1495

1083

1412

1529

Properties and Uses

The most widely used of the light metals.

A brittle, crystalline metal which, however, isused in bearings and type.

An inert gas present in small amounts in theatmosphere. Used in 'argon-arc' welding.

A black crystalline element—used to hardencopper at elevated temperatures.

Its compounds are useful because of theirfluorescent properties.

A light metal which is used to strengthencopper. Also used un-alloyed in atomic-energyplant.

A metal similar to antimony in many ways—used in the manufacture of fusible(low-melting-point) alloys.

Known chiefly in the form of its compound,'borax'.

Used for plating some metals and alloys andfor strengthening copper telephone wires.

A very reactive metal met chiefly in the formof its oxide, 'quicklime'.

The basis of all fuels and organic substancesand an essential ingredient of steel.

A 'rare-earth' metal. Used as an 'inoculant' incast iron, and in the manufacture of lighterflints.

A poisonous reactive gas, used in thede-gasification of light alloys.

A metal which resists corrosion—hence it isused for plating and in stainless steels andother corrosion-resistant alloys.

Used chiefly in permanent magnets and inhigh-speed steel.

A metal of high electrical conductivity which isused widely in the electrical industries and inalloys such as bronzes and brasses.

Present in the 'rare earths', used in somemagnesium-base alloys.

A silvery-white metal. Present in the 'rareearths', used in some magnesium alloys andalso used in cancer-therapy generators.

Table 1.1 {continued)

Element

Gadolinium

Gold

Helium

Hydrogen

Indium

lridium

Iron

Lanthanum

Lead

Magnesium

Manganese

Mercury

Molybdenum

Neodymium

Nickel

Niobium

Nitrogen

Osmium

Symbol

Gd

Au

He

H

In

Ir

Fe

La

Pb

Mg

Mn

Hg

Mo

Nd

Ni

Nb

N

Os

Relativeatomicweight(C =12.0000)

157.25

196.967

4.0026

1.00797

114.82

192.2

55.847

138.91

207.69

24.312

54.938

200.59

95.94

144.24

58.71

92.906

14.0067

190.2

RelativeDensity(SpecificGravity)

7.9

19.3

0.16 x10~3

0.09 x10"3

7.3

22.4

7.9

6.1

11.3

1.7

7.2

13.6

10.2

7.0

8.9

8.6

1.16 x10~3

22.5

MeltingpointCC)

1313

1063

Below-272

-259

156.6

2454

1535

920

327.4

651

1260

-38.8

2620

1021

1458

1950

-210

2700

Properties and Uses

Used in some modern permanent magnetalloys. Also present in the 'rare earths' usedin some magnesium-base alloys.

Of little use in engineering, but mainly as asystem of exchange and in jewellery.

A light non-reactive gas present in smallamounts in the atmosphere.

The lightest element and a constituent of mostgaseous fuels.

A very soft greyish metal used as acorrosion-resistant coating also used in somelow melting point solders, and insemiconductors.

A heavy precious metal similar to platinum.

A fairly soft white metal when pure, but rarelyused thus in engineering.

Used in some high-temperature alloys.

Not the densest of metals, as the metaphor'as heavy as lead' suggests.

Used along with aluminium in the lightest ofalloys.

Similar in many ways to iron and widely usedin steel as a deoxidant.

The only liquid metal at normal temperatures—known as 'quicksilver'.

A heavy metal used in alloy steels.

A yellowish-white metal. Used in someheat-treatable magnesium-base alloys and insome permanent magnet alloys.

An adaptable metal used in a wide variety offerrous and non-ferrous alloys.

Used in steels and, un-alloyed, inatomic-energy plant. Formerly called'Columbium' in the United States.

Comprises about 4/5 of the atmosphere. Canbe made to dissolve in the surface of steeland so harden it.

The densest element and a rare white metallike platinum.

Table 1.1 {continued)

Element

Oxygen

Palladium

Phosphorus

Platinum

Potassium

Rhodium

Samarium

Selenium

Silicon

Silver

Sodium

Strontium

Sulphur

Tantalum

Tellurium

Thallium

Thorium

Symbol

O

Pd

P

Pt

K

Rh

Sm

Se

Si

Ag

Na

Sr

S

Ta

Te

Tl

Th

Relativeatomicweight(C =12.0000)

15.999

106.4

30.9738

195.09

39.102

102.905

150.35

78.96

28.086

107.87

22.9898

87.62

32.064

180.948

127.6

204.37

232.038

RelativeDensity(SpecificGravity)

1.32 x10"3

12.0

1.8

21.4

0.86

12.4

7.5

4.8

2.4

10.5

0.97

2.6

2.1

16.6

6.2

11.85

11.2

MeltingpointCC)

-218

1555

44

1773

62.3

1985

1077

220

1427

960

97.5

772

113

3207

452

303

1850

Properties and Uses

Combined with other elements it comprisesnearly 50% of the Earth's crust and 20% of theEarth's atmosphere in the uncombined state.

Another platinum-group metal.

A reactive element; in steel it is a deleteriousimpurity, but in some bronzes it is anessential addition.

Precious white metal used in jewellery and inscientific apparatus because of its highcorrosion resistance.

A very reactive metal which explodes oncontact with water.

A platinum-group metal used in themanufacture of thermocouple wires.

A light grey metal. Used in samarium-cobaltpermanent magnets (electronic watches).

Mainly useful in the manufacture ofphoto-electric cells.

Known mainly as its oxide, silica (sand, quartz,etc.), but also, in the elemental form, in castirons, some special steels and non-ferrousalloys.

Widely used for jewellery and decorative work.Has the highest electrical conductivity of anymetal—used for electrical contacts.

A metal like potassium. Used in the treatmentof some of the light alloys.

Its compounds produce the red flames infireworks. An isotope 'Strontium 90' present inradioactive 'fall out'.

Present in many metallic ores—thesteelmaker's greatest enemy.

Sometimes used in the manufacture ofsuper-hard cutting tools of the'sintered-carbide' type. Also unalloyed wherehigh corrosion resistance is necessary(chemical plant).

Used in small amounts to strengthen lead.

A soft heavy metal forming poisonouscompounds.

A rare metal—0.75% added to tungstenfilaments (gives improved electron emission).

Table 1.1 (continued)

Element

Tin

Titanium

Tungsten

Uranium

Vanadium

Ytterbium

Yttrium

Zinc

Zirconium

Symbol

Sn

Ti

W

U

V

Yb

Y

Zn

Zr

Relativeatomicweight(C =12.0000)

118.69

47.9

183.85

238.03

50.942

173.04

88.9

65.37

91.22

RelativeDensity(SpecificGravity)

7.3

4.5

19.3

18.7

5.7

7.0

4.5

7.1

6.4

Meltingpoint(0C)

231.9

1725

3410

1150

1710

817

1522

419.5

1800

Properties and Uses

A widely used but rather expensive metal. Tincans' carry only a very thin coating of tin onmild steel.

Small additions are made to steels andaluminium alloys to improve their properties.Used in the alloyed and unalloyed form in theaircraft industry.

Imparts very great hardness to steel and is themain constituent of 'high-speed' steel. Its highm.pt. makes it useful for lamp filaments.

Used chiefly in the production of atomicenergy.

Added to steels as a 'cleanser' (deoxidiser)and a hardener.

Present in the 'rare earths' used in somemagnesium-base alloys. A silvery-white metal.

Used in heat-treatable magnesium-basealloys. Also in some high-temperature alloys.

Used widely for galvanising mild steel and alsoas a basis for some die-casting alloys. Brassesare copper-zinc alloys.

Small amounts used in magnesium andhigh-temperature alloys. Also, un-alloyed, inatomic energy plant, and in some chemicalplant because of good corrosion resistance.

gen is by far the most common element. In fact it accounts for some 90%of the total matter therein, with helium representing most of the remaining10%; all other elements being restricted to no more than 0.2% in total.Magnesium, at about 0.003% is possibly the most abundant metal in theUniverse. We are indeed fortunate in the variety and abundance of chemi-cal elements on Earth.

Chemical Reactions and Equations

1.30 Readers will be familiar with the behaviour of the chemical sub-stances we call salts during electrolysis. The metallic particles (or ions asthey are termed) being positively charged are attracted to the negative

electrode (or cathode), whilst the non-metallic ions being negativelycharged are attracted to the positive electrode (or anode). Because of thebehaviour of their respective ions metals are said to be electropositive andnon-metals electronegative.

In general elements react or combine with each other when they possessopposite chemical natures. Thus the more electropositive a metal the morereadily will it combine with a non-metal, forming a very stable compound.If one metal is more strongly electropositive than another the two maycombine forming what is called an intermetallic compound, though usuallythey only mix with each other forming what is in effect a 'solid solution'.This constitutes the basis of most useful alloys.

1.31 Atoms combine with each other in simple fixed proportions. Forexample, one atom of the gas chlorine (Cl) will combine with one atom ofthe gas hydrogen (H) to form one molecule of the gas hydrogen chloride.We can write down a formula for hydrogen chloride which expresses at aglance its molecular constitution, viz. HCl. Since one atom of chlorine willcombine with one atom of hydrogen, its valence is said to be one, the termvalence denoting the number of atoms of hydrogen which will combinewith one atom of the element in question. Again, two atoms of hydrogencombine with one atom of oxygen to form one molecule of water (H2O),so that the valence of oxygen is two. Similarly, four atoms of hydrogenwill combine with one atom of carbon to form one molecule of methane(CH4). Hence the valence of carbon in this instance is four. However,carbon, like several other elements, exhibits a variable valence, since it willalso form the substances ethene (C2H4), formerly 'ethylene', and ethine(C2H2), formerly acetylene.

1.32 Compounds also react with each other in simple proportions, andwe can express such a reaction in the form of a chemical equation thus:

CaO + 2HCl = CaCl2 + H2O

Table 1.2 The Approximate Composition of the Earth's Crust (to the Extent of Mining Operations)

Element

OxygenSiliconAluminiumIronCalciumSodiumPotassiumMagnesiumHydrogenTitaniumCarbonChlorinePhosphorusManganeseSulphurBariumNitrogenChromium

% by mass

49.126.0

7.44.33.22.42.32.31.00.610.350.200.120.100.100.050.040.03

Element

NickelVanadiumZincCopperTinBoronCobaltLeadMolybdenumTungstenCadmiumBerylliumUraniumMercurySilverGoldPlatinumRadium

% by mass

0.020.020.020.010.0080.0050.0020.0020.001

9 x 10"4

5 x 10"4

4 x 10"4

4 x 10"4

1 x 10"4

1 x 10~5

5 x 10"6

5 x 10"6

2 x 1O~10

The above equation tells us that one chemical unit of calcium oxide (CaOor 'quicklime') will react with two chemical units of hydrogen chloride(HCl or hydrochloric acid), to produce one chemical unit of calcium chlor-ide (CaCb) and one chemical unit of water (H2O). Though the totalnumber of chemical units may change due to the reaction—we began withthree units and ended with two—the total number of atoms remains thesame on either side of the equation. The equation must balance rather likea financial balance sheet.

1.33 By substituting the appropriate atomic masses (approximatedvalues from Table 1.1) in the above equation we can obtain further usefulinformation from it.

Thus, 56 parts by weight of quicklime will react with 73 parts by weight ofhydrogen chloride to produce 111 parts by weight of calcium chloride and18 parts by weight of water. Naturally, instead of 'parts by weight' we canuse grams, kilograms or tonnes as required.

We will now deal with some chemical reactions relevant to a study ofmetallurgy.

Oxidation and Reduction

1.40 Oxidation is one of the most common of chemical processes. Itrefers, in its simplest terms, to the combination between oxygen and anyother element—a phenomenon which is taking place all the time aroundus. In our daily lives we make constant use of oxidation. We inhaleatmospheric oxygen and reject carbon dioxide (CO2)—the oxygen webreathe combines with carbon from our animal tissues, releasing energy inthe process. We then reject the waste carbon dioxide. Similarly, heatenergy can be produced by burning carbonaceous materials, such as coalor petroleum. Just as without breathing oxygen animals cannot live, sowithout an adequate air supply fuel cannot burn. In these reactions carbonand oxygen have combined to form a gas, carbon dioxide (CO2), and atthe same time heat energy has been released—the 'energy potential' ofthe carbon having fallen in the process.

1.41 Oxidation, however, is also a phenomenon which works to ourdisadvantage, particularly in so far as the metallurgist is concerned, sincea large number of otherwise useful metals show a great affinity for oxygenand combine with it whenever they are able. This is particularly so at hightemperatures so that the protection of metal surfaces by means of fluxesis often necessary during melting and welding operations. Although cor-rosion is generally a more complex phenomenon, oxidation is always

involved and expensive processes such as painting, plating or galvanisingmust be used to protect the metallic surface.

1.42 It should be noted that, to the chemist, the term oxidation has amuch wider meaning, and in fact refers to a chemical process in which theelectronegative (or non-metallic) constituent of the molecule is increased.(1.74) For example, iron (II) chloride may be oxidised to iron (III)chloride—

2FeCl2 + Cl2 = 2FeCl3Iron(II) Iron(III)chloride chloride

The element oxygen is not involved in this reaction, yet we say that iron(II) chloride has been 'oxidised' by chlorine since the chlorine ion iselectronegative and so the electronegative portion of the iron (III) chlor-ide molecule is greater than that of the iron (II) chloride molecule.

Since iron exhibits valences of both two and three this is indicated, incurrent chemical nomenclature, by writing either iron (II)' or 'iron (III)'as appropriate. Formerly the terms 'ferrous' or 'ferric' were used todescribe these two series of compounds. In fact any metal exhibiting avariable valence gave rise to '-ous' and '-/c' series of compounds; -ousbeing used to describe that series in which the metal exhibits the lowervalence and -ic that in which the metal exhibits the higher valence.

1.43 Whilst many metals exist in the Earth's crust in combination withoxygen as oxides, others are combined with sulphur as sulphides. The latterform the basis of many of the non-ferrous (that is, containing no iron)metal ores. The separation and removal of the oxygen or sulphur containedin the ore from the metal itself is often a difficult and expensive process.Most of the sulphide ores are first heated in air to convert them to oxides,eg—

2ZnS + 3O2 = 2ZnO + 2SO2Zinc sulphide Zinc oxide Sulphur('zinc blende') dioxide (gas)

The oxide, whether occurring naturally or produced as indicated in theabove equation, is then generally mixed with carbon in the form of cokeor anthracite and heated in a furnace. In most cases some of the carbon isburned simultaneously in order to provide the necessary heat which willcause the reaction to proceed more quickly. Under these conditions carbonusually proves to have a greater affinity for oxygen than does the metaland so takes oxygen away from the metal, forming carbon dioxide andleaving the metal (often impure) behind, eg—

2ZnO-HC = 2Zn+ CO2

This process of separating the atoms of oxygen from a substance is knownas reduction. Reduction is thus the reverse of oxidation, and again, in the

wider chemical sense, it refers to a reaction in which the proportion of theelectronegative constituent of the molecule is decreased.

1.44 Some elements have greater affinities for oxygen than have others.Their oxides are therefore more difficult to decompose. Aluminium andmagnesium, strongly electropositive metals, have greater affinities for oxy-gen than has carbon, so that it is impossible to reduce their oxides in thenormal way using coke—electrolysis, a much more expensive process,must be used. Metals, such as aluminium, magnesium, zinc, iron and lead,which form stable, tenacious oxides are usually called base metals, whilstthose metals which have little affinity for oxygen are called noble metals.Such metals include gold, silver and platinum, metals which will not scaleor tarnish to any appreciable extent due to the action of atmosphericoxygen.

Acids, Bases and Salts

1.50 When an oxide of a non-metal combines with water it forms whatwe call an acid. Thus sulphur trioxide (SO3) combines with water to formthe well-known sulphuric acid (H2SO4), and sulphur trioxide is said to bethe anhydride of sulphuric acid. Though not all acids are as corrosive assulphuric, it is fairly well known that in cases of accident involving acidsof this type it is necessary to neutralise the acid with some suitable antidote.

1.51 Substances which have this effect are called bases. These are met-allic oxides (and hydroxides) which, when they react with an acid, producewater and a chemical compound which we call a salt. Typical examples ofthese acid-base reactions are—

H2SO4 + CaO = CaSO4 + H2OSulphuric Calcium oxide Calcium Water

acid ('quicklime') sulphate(a salt)

HCl +NaOH= NaCl +H2OHydrochloric Sodium Sodium

acid hydroxide chloride('caustic ('commonsoda') salt')

We can generalise in respect of equations like this and say—

Acid + Base = Salt + Water

Similarly, the acid anhydride will often combine with a base, forming asalt, eg—

SO3 + CaO = CaSO4

This type of reaction occurs quite frequently during the smelting of metallicores.

1.52 One of the most common elements in the Earth's crust is silicon,present in the form of its oxide, silica (SiO2). Since silicon is a non-metal,its oxide is an acid anhydride, and though all the common forms of silica,such as sand, sandstone and quartz, do not seem to be of a very reactivenature at normal temperatures, they are sufficiently reactive when heatedto high temperatures to combine with many of the metallic oxides (whichare basic) and produce neutral salts called silicates. Silica occurs entangledwith most metallic ores, and although some of it is rejected by mechanicalmeans before the ore is charged to the furnace, some remains and couldconstitute a difficult problem in that its high melting point of 1780° C wouldcause a sort of 'indigestion' in the furnace. To overcome this a sufficientquantity of a basic flux is added in order to combine with the silica andproduce a slag with a melting point low enough to allow it to run from thefurnace. The cheapest metallic oxide, and the one in general use, is lime.

SiO2 +2CaO = 2CaO. SiO2Acid Base Salt (calcium

anhydride silicate)

The formula for the slag, calcium silicate, is generally written 2CaO.SiO2

rather than Ca2SiO4, since lime and silica will combine in other pro-portions. When the formula is written in the former manner, the reactingproportions of silica and lime can be seen at a glance.

1.53 On the other hand acid/basic reactions can constitute a problemwhen they involve similar reactions between slags and furnace linings.Thus, since we do not wish to liquefy our furnace lining, we must makesure that it does not react with the charge or the slag covering it. In short,we must make sure that the furnace lining is of the same chemical natureas the slag, ie if the slag contains an excess of silica, and is therefore acid,we must line the furnace with a similar silica-rich refractory, such as silicabrick or ganister; whilst if the slag contains an excess of lime or other basicmaterial, we must line the furnace with a basic refractory, such as burntdolomite (CaO.MgO) or burnt magnesite (MgO). If the chemical natureof the furnace lining is the same as that of the slag, then, clearly, noreaction is likely to take place between them. Since silica and ganister, onthe one hand, and dolomite and magnesite, on the other, all have highsoftening temperatures, they will also be able to resist the high tempera-tures encountered in many of the metallurgical smelting processes.

Atomic Structure

1.60 It was the Ancient Greek philosopher Leukippos and his discipleDemokritos who, during the fifth century BC, suggested that if matterwere progressively subdivided a point would be reached where furthersubdivision was impossible. The Greek word for 'indivisible' is 'atomos'.More than 2000 years later—in 1808—a British chemist, John Daltonannounced his Atomic theory which was based on the original Leukippos/

Demokritos idea. Dalton suggested that chemical reactions could beexplained if it was assumed that each chemical element consisted ofextremely small indivisible particles, which, following the Greek concept,he called 'atoms'. The Theory was generally accepted but before the endof the nineteenth century it was discovered that atoms were certainly notindivisible. Thus the 'atom' is an ill-described particle. But the title becameso firmly established that it is retained to-day, though we usually modifyour description to add that it is 'the smallest stable particle of matter whichcan exist'.

1.61 In 1897 the English physicist J. J. Thomson showed that a beamof 'cathode rays' was in fact a stream of fast-moving negatively chargedparticles—they were in fact electrons. Because the electron is negativelycharged it can be deflected from its path by an electrical field and we makeuse of this feature in TV tubes where a beam of electrons, deflected by asystem of electro-magnetic fields, builds up a picture by impingement ona screen which will fluoresce under the impact of electrons. Thomson wasable to make only a rough estimate of the mass of the electron but wasable to show that it was extremely small compared with a hydrogen atom.Thus Dalton's 'indivisible atom' fell apart.

1.62 Since atoms are electrically neutral—common everyday materialscarry no resultant electrical charge—the discovery of the negativelycharged electron stimulated research to find a positively charged particle.This was ultimately discovered in the form of the nucleus of the hydrogenatom, with a mass some 1837 times greater than that of the electron butwith an equal but opposite positive charge. In 1920 the famous New Zea-land born physicist Ernest Rutherford suggested it be called the proton.

So, atoms were assumed to be composed of equal numbers of electronsand protons, the electrons being arranged in 'shells' or 'orbits' around abunch of protons which constituted the nucleus of the atom. But there wasa snag: the true atomic weights of the elements, which had been derivedby careful independent experiment over many years, were much greaterthan the atomic weights calculated from an assumption of the numbers ofprotons and electrons present in the atom of a particular element. Chemistsexplained this 'dead weight' in the atomic nucleus by suggesting that therewere extra protons in the nucleus which had been 'neutralised' electricallyby the presence of electrons also lurking there. Thus in the early 1930selectrons were classed as being either 'planetary' or 'nuclear.'

1.63 At about this time the English physicist Sir James Chadwick dis-covered the neutron, a particle of roughly the same mass as the proton butcarrying no electrical charge. Its presence in the atomic nucleus made iteasier to explain that part of the atomic mass not attributable to a simpleelectron/proton balance. Its electrical neutrality made it a useful particlein atomic research, since it could be fired into a nucleus without beingrepelled by like electrical charges. Chadwick's discovery did in fact alterthe course of history since it made possible the development of the AtomicBomb ten years later (18.74).

1.64 Since Chadwick's time the number of elementary particles hasproliferated. These can be classified into three main groups:

1 Baryons (protons and other particles with a mass greater than that ofthe proton).

2 Mesons (any of a group of particles with a rest mass between those ofthe electron and proton, and with an integral 'spin').

3 Leptons (among which are the electron, positron or 'positive electron'and the neutrino which possesses neither charge nor mass but only'spin').

The term 'quark' is used to describe any one of a number of hypotheticalelementary particles with charges of % or -1A of the electron charge, andthought to be fundamental units of all baryons and mesons. It is interestingto note that the word 'quark' was devised by James Joyce in Finnegan'sWake. Indeed, to date the existence of more than two hundred differentsubatomic particles has been reported. Of these only three—electron, pro-ton and neutron—appear to have any substantial influence on the distinc-tive properties of each element. Consequently the rest will receive nofurther mention in this book. The essential features of electron, protonand neutron are summarised in Table 1.3. Since the real mass of theseparticles is inconveniently small for calculations their masses relative tothat of the carbon atom (isotope C= 12) are generally used.

Table 1.3

Relative massParticle Actual rest mass (kg) (12C= 12) Charge (C)

Electron 9.11 x 10~31 0.000 548 8 -1.602 x 10~19 \ Equal but oppositeProton 1.672 x 10"27 1.007 263 +1.602 x 10"19 f

Neutron 1.675 x 1O"27 1.008 665 0 Zero

1.65 In the early days of the twentieth century Rutherford carried outa series of classical experiments in which he fired a-particles—the nucleiof helium atoms and therefore positively charged—at very thin gold foil.Most of these particles passed right through the foil whilst about one in20 000 rebounded along its incident path. Others were deflected at variousangles to the incident beam. From these experiments Rutherford con-cluded that the atom consisted of a comparatively small nucleus containingprotons, around which circulated electrons. This concept of the atom wasdeveloped by the Danish physicist Niels Bohr. He proposed that the elec-trons were placed in a series of fixed orbits which varied in number withthe complexity of the atom. The constitution of an atom was thus regardedas being similar to that of the solar system and containing about the samedensity of actual 'matter'. This model was later modified to the extent thatthe definite electron orbit was replaced by a mathematical function whichrepresents the distribution of 'electrons' in the space occupied by the atom.This distribution is referred to as the orbital of the electron. In fact manyvisualise the electron as being in the nature of a 'cloud' of electricity ratherthan as a discrete particle, the orbital indicating the density of that cloudat any point within the atom. From the point of view of any diagramrepresenting atomic structure it is more convenient to indicate the electron

as being a definite particle travelling in a simple circular orbit round anucleus consisting of protons and neutrons, but diagrams such as Fig. 1.1should be studied with this statement in mind. Fig. 1.1 in no way representswhat atoms 'look like'.

1.66 The most simple of all atoms is that of ordinary hydrogen. Itconsists of one proton with one electron in orbital around it. Since thepositive charge of the proton is balanced by the equal but negative chargeof the electron, the resultant atom will be electrically neutral. The mass ofthe electron being very small compared with that of the proton, the massof the atom will be roughly that of the proton.

1.67 An atom of ordinary helium comes next in order of both massand complexity. Here the nucleus contains two protons which are associ-ated with two electrons in the same 'shell', ie similar orbitals surroundingthe nucleus. The nucleus also contains two neutrons. However, in Table1.4, which indicates the proton-electron make-up of some of the simpleratoms, neutrons have been omitted for reasons which will become apparentlater (1.90). The number of protons in the nucleus, which is equal to thetotal number of electrons in successive shells, is called the Atomic numberof the element.

In Table 1.4 it will be noted that with the metal lithium a new electronshell is formed and that this 'fills up' by the addition of a single electronwith each successive element until, with the 'noble'* gas neon, it containsa total of eight electrons. With the metal sodium another new shell thenbegins and similarly fills so that with the noble gas argon this third shellalso contains eight electrons. The next shell then begins to form with themetal potassium.

1.68 In the case of the elements dealt with in Table 1.4, this periodicityin respect of the number of electrons in the outer shell is reflected in thechemical properties of the elements themselves. Thus the metals lithium,sodium and potassium each have a single electron in the outer shell andall are very similar chemically. They will all oxidise very rapidly and reactreadily with water, liberating hydrogen and forming soluble hydroxides.Each of these elements has a valence of one. Physically, also, they are verysimilar in that they are all light soft metals, more or less white in colour.

In a similar way the gases fluorine and chlorine, with seven electrons inthe outer shell in each case, have like chemical properties. Both arecoloured gases (at normal temperatures and pressures) with strongly non-metallic properties.

The noble gases helium, neon and argon occur in small quantities in theatmosphere. In fact it is only there where they are likely to exist undernatural conditions, since these noble gases are similar in being non-reactive and, under ordinary circumstances, unable to combine with otherelements. Chemical combination between elements is governed by thenumber of electrons present in the outer shell of each atom concerned.When the outer shell contains eight electrons it becomes, as it were, 'satu-

* In the chemical sense the term 'noble' means that an element is not very reactive—thus, the 'noble'metals, gold, platinum, etc., are not readily attacked by other reactive substances, such as corrosive acids.

POTASSIUMC 2,80,1 J

Fig. 1.1 This diagram indicates the electron-proton make-up of the nineteen simplest atoms, but it does not attemptto illustrate the manner in which they are actually distributed.

ARGON[2 8.B]

CHLORINEC 2,8,7)

SULPHURC 2,8,6)

NEONC 2,8)

HELIUM(2)

FLUORINEC2.7)

O X Y G E NC 2.61

NITROGENC 2 . 5 )

PHOSPHORUSC'2,8,5)

SILICONC 2,8,4]

CARBONC 2,4)

BORONC 2,3)

ALUMINIUMC 2.8,3)

MAGNESIUM(2.8,2)

SODIUM[2,8.I)

LITHIUM[2,1 ]

BERYLLIUMC2.2)

HYDROGENCU

rated', so that such an atom will have no tendency to combine with others.1.69 The periodicity of properties described above was noticed by

chemists quite early in the nineteenth century and led to the advent oforder in inorganic chemistry with the celebrated 'Periodic Classification ofthe Elements' by the Russian chemist Demitri Mendeleef in 1864. In morerecent years this periodicity in chemical properties of the elements hasbeen explained in terms of the electronic structure of the atom as outlinedvery briefly above.

In elements with atomic numbers greater than that of potassium, morecomplex shells containing more than eight electrons are present in theatom. These more complex shells are divided into sub-shells and some'overlapping' of orbitals occurs so that new sub-shells tend to formbefore a previous shell has been 'filled' (Table 1.4). This gives rise tothe 'transition metals' situated roughly in the centre of modern versionsof the Periodic Table (Fig. 1.2). Nevertheless, elements in the verticalcolumns—or 'Groups'—have similar electron structures and thereforesimilar properties.

4f is not filleduntil elementno.7l(Lu) bywhich point 6shas alreadybeen filled.

4s begins tofill before 3d

sub-shells

Shell 4

sub-shells

Shell 3

sub shells

Shell 2

Shell IAtomicNumberElement

Table 1.4 Electron Notation of the Elements in the First Four Periods of the Periodic Classification

Chemical Combination and Valence

1.70 It was mentioned above that chemical combination between twoatoms is governed by the number of electrons in the outer electron shellof each. Moreover, it was pointed out that those elements whose atomshad eight electrons in the outer shell (the noble gases neon and argon) hadno inclination to combine with other elements and therefore had no chemi-cal affinity. It is therefore reasonable to suppose that the completion ofthe 'octet' of electrons in the outer shell of an atom leads to a valence ofzero. The noble gas helium, with a completed 'duplet' of electrons in thesingle shell, behaves in a similar manner.

As far as the simpler atoms we have been discussing are concerned thetendency is for them to attempt to attain this noble-gas structure of a stableoctet (or duplet) of electrons in the outer shell. Their chemical propertiesare reflected in this tendency. With the more complex atoms the situationis not quite so simple, since these atoms possess larger outer shells whichare generally sub-divided, to the extent that electrons may begin to fill anew outer 'sub-shell' before the penultimate sub-shell has been completed.As mentioned above this would explain the existence of groups of metallicelements the properties of which are transitional between those of onewell-defined group and those of the next. The broad principles of theelectronic theory of valence mentioned here in connection with the simpleratoms will apply. On these general lines three main forms of combinationexist.1.71 Electro-valent Combination In this type of combination a metallicatom loses the electrons which constitute its outer shell (or sub-shell) andthe number of electrons so lost are equivalent to the numerical valence ofthe element. These lost electrons are transferred to the outer electron shellsof the non-metallic atom (or atoms) with which the metal is combining. Inthis way a complete shell of electrons is left behind in the metallic particlewhilst a hitherto incomplete shell is filled in the non-metallic particle.

Let us consider the combination which takes place between the metalsodium and the non-metal chlorine to form sodium chloride (commonsalt). The sodium atom has a single electron in its outer shell and thistransfers to join the seven electrons in the outer shell of the chlorine atom.When this occurs each resultant particle is left with a complete octet in theouter shell. (The sodium particle now has the same electron structure as thenoble gas neon, and the chlorine particle has the same electron structure asthe noble gas argon.) The balance of electrical charges which existedbetween protons and electrons in the original atoms is, however, upset.Since the sodium atom has lost a negatively charged particle (an electron),the remaining sodium particle must now possess a resultant positive charge.Meanwhile the chlorine atom has gained this electron so the resultantchlorine particle must carry a negative charge. These charged particles,derived from atoms in this manner, are called ions. In terms of symbolsthe sodium ion is written thus, Na+, and the chlorine ion, Cl".

1.72 Since sodium ions and chlorine ions are oppositely charged they

Fig. 1.2 The periodic classification of the elements.All elements with atomic numbers above 92 are 'artificial'—the products of the nuclear scientist. Since this classificationwas last revised numbers 107 (Uns), 108 (Uno) and 109 (Une) have been reported. Soviet scientists are claiming 110(Uun). Fortunately the fashion for assigning 'patriotic' names to these relatively unimportant metals is now past, andIUPAC (the International Union of Pure and Applied Chemistry) allocates to each new element a name which statesits atomic number in 'Dog Latin'. Thus 'Unq' (104) is 'Unnilquadum', ie Un-nil-quadum or 1-0-4; whilst 'Uun' (110) is'Ununnilium', ie Un-un-nilium or 1-1-0.

NOBLEGASES

COMPLETELY FILLEDNOBLE GAS 'SHELLS'

NON-METALSMETALS

TRANSITION METALS

lanthanides

actinides

Fig. 1.3 The formation of the electro-valent bond in sodium chloride, by the transfer ofan electron from the sodium atom to the chlorine atom.

will attract each other and the salt sodium chloride crystallises in a simplecubic form in which sodium ions and chlorine ions arrange themselves inthe manner indicated in Fig. 1.4. Except for the force of attraction whichexists between oppositely charged particles, no other 'bond' exists betweensodium ions and chlorine ions, and when a crystal of sodium chloride isdissolved in water separate sodium and chlorine ions are released and canmove as separate particles in solution. Such a solution is known as anelectrolyte because it will conduct electricity. If we place two electrodesinto such a solution and connect them to a direct-current supply, the posi-tively charged sodium ions will travel to the negative electrode and thenegatively charged chlorine ions will travel to the positive electrode. Theapplied EMF does not 'split up' the sodium chloride—the latter ionises assoon as it dissolves in water.

1.73 Thus, the unit in solid sodium chloride is the crystal, whilst insolution separate ions of sodium and chlorine exist. In reality there is nosodium chloride molecule and it is therefore incorrect to express the saltas 'NaCl'. Busy chemists are, however, in the habit of using symbols inthis manner as a type of chemical shorthand. The author has in fact beenguilty of this indiscretion earlier in this chapter when discussing formulaeand equations in which electro-valent compounds are involved. Forexample, the equation representing the reaction between hydrochloric acidand caustic soda (1.51) would more correctly be written:

BEFOREREACTION

17PROTONS

IlPROTONS

Il PROTONSIl ELECTRONSU,8,IJ

CHLORINE ATOM17 PROTONS17 ELECTRONS (2,8,7^

AFTERREACTION

CHLORINE ION17 PROTONSIB ELECTRONS(2,8,8J

-HENCE A RESULTANTNEGATIVE CHARGE.

Il P ROTONSIO ELECTRONS (2,8)

-HENCE A RESULTANTPOSITIVE CHARGE.

17PROTONS

I IPROTONS

Fig. 1.5 (i) A solution of sodium chloride in which the separate sodium ions and chlorineions are moving independently within the solution. Note that ionisation of the salt has takenplace on solution and does not depend upon the passage of an electric current, (ii) WhenEMF is applied to the solution the charged ions are attracted to the appropriate electrode.

H+ +Cl" + N a N 1 O H " - Na+ +Cl" +H2OHydrochloric Sodium Sodium

Acid hydroxide chloride

1.74 It was suggested earlier in this chapter (1.40) that the term 'oxida-tion' had a wider meaning in chemistry than the combination of an elementwith oxygen. Thus, when metallic iron combines with the gas chlorine to

SODIUM ION

CHLORINE ION

Fig. 1.4 A simple cubic crystal lattice such as exists in solid sodium chloride.

SODIUM IONCHLORINE ION

ELECTRODES

(i) (ii)

form iron(//) chloride, FeCl2, the iron is said to have been oxidised, whilstwhen the iron(II) chloride so produced combines with still more chlorineto form iron(///) chloride, FeCl3, the iron(II) chloride in turn has beenoxidised. At each stage the 'electronegative portion' of the substance hasincreased. We can now relate this process of oxidation to a transfer ofelectrons. In being oxidised, an atom of iron has lost electrons to becomean ion—first the iron(//) ion, Fe++, at which stage it has lost two electronsand then the iron(///) ion, Fe+++ , when it has lost three electrons to thechlorine atoms:

Fe + Cl2-+Fe++ +2Cr-^>Fe + + + +3Cl"iron(II) chloride iron(III) chloride

Thus oxidation of a substance, in this case iron, involves a loss of electronsby atoms of that substance.

Conversely, reduction involves a gain of electrons. For example, wheniron(III) oxide, Fe2O3, is reduced to metallic iron in the blast furnace theFe+++ ion receives electrons and becomes an atom of iron:

Fe+ + + + 3 e - > F e

1.75 Covalent Combination In this type of chemical combination thereis no loss' of electrons from one atom to another. Instead a certain numberof electrons are 'shared' between two or more atoms to produce a stableparticle which we call a molecule. In a molecule of the gas methane, fourhydrogen atoms are combined with one carbon atom. The carbon atomhas four electrons in its outer shell, but these are joined by four moreelectrons, contributed singly by each of the four hydrogen atoms (Fig. 1.6).Thus the octet of the carbon atom is completed and at the same time, bysharing one of the carbon atom's electrons, each hydrogen atom is able tocomplete its 'helium duplet'. This sharing of electrons by two atoms bindsthem together, and a molecule is formed in which atoms are held togetherby strong valence bonds. Each shared electron now passes from an orbitalcontrolled by one nucleus into an orbital controlled by two nuclei and it isthis control which constitutes the covalent bond. Chemists express thestructural formula for the methane molecule thus:

H

H - C - HI

HEach co-valent bond is indicated so: —. Co-valent compounds, sincethey do not ionise, will not conduct electricity and are therefore non-electrolytes. They include many of the organic* compounds, such as ben-zene, alcohol, turpentine, chloroform and members of the 'alkane' series.

As the molecule size of co-valent compounds increases, so the bond

* Those compounds associated with animal and vegetable life and containing mainly the elementscarbon, hydrogen and oxygen.

Fig. 1.6 (i) Co-valent bonding in a molecule of methane, CH4. (ii) Spatial arrangement ofatoms in the methane molecule.

strength of the material increases, as indicated in complex compounds suchas rubber and vegetable fibres. Sometimes simple molecules of co-valentcompounds can be made to unite with molecules of their own type, forminglarge chain-type molecules in which the bond strength is very high. Thisprocess is called 'polymerisation'. For example, the gas ethene C2H4 canbe made to polymerise forming polythene:

ETHENE POLY-ETHENE (POLYTHENE^

——APPROX- I 2OO CARBON ATOMS

CARBON ATOMHYDROGEN ATOM

As the molecular chain increases in length, the strength also increases.Nylon (synthesised from benzene) and polychloroethene* (synthesisedfrom ethine*) are both 'super polymers', the strength of which dependsupon a long chain of carbon atoms co-valently bonded.

Forces of attraction (1.80), acting between points where these 'chainmolecules' touch each other, hold the mass of them together. If such asubstance is heated, the forces acting between the molecules are reducedand, under stress, the fibrous molecules will gradually slide over eachother into new permanent positions. The substance is then said to bethermoplastic. Substances which (like water) are built up of simple mol-ecules generally melt at a sharply defined single temperature since there isno entanglement, as exists with the ungainly molecules of the super poly-mers, and when the forces of attraction between simple molecules fallbelow a certain limit they can separate instantaneously. Super polymers,then, soften progressively as the temperature is increased rather than meltat a well-defined temperature.

Some polymers, when heated, undergo a chemical change which firmlyanchors the chain molecules to each other by means of co-valent bonds.

* Formerly 'polyvinyl chloride' (PVC) and 'acetylene' respectively.

H ATOM

CATOM(ii)(i)

On cooling, the material is rigid and is said to be thermosetting. Bakeliteis such a substance.

Rubber possesses elasticity (4.11) by virtue of the folded nature of itslong-chain molecules. When stressed, one of these molecules will extendafter the fashion of a spiral spring and, when the stress is removed, it willreturn to its original shape. In raw rubber the tensile stress will also causethe chain molecules to slide relative to each other, so that, when the stressis removed, some permanent deformation will remain in the material (Fig.1.7). If the raw rubber is mixed with sulphur and heated it becomes 'vulcan-ised'. That is, sulphur causes the formation of co-valent links between thelarge rubber molecules which are thus held firmly together. Consequently,vulcanised rubber possesses elasticity due to the behaviour of its foldedchain molecules, but it resists permanent deformation, since these mol-ecules are no longer able to slide over each other into new positions.

Fig. 1.7. Rubber molecules are of the long-chain type.Due to their 'folded' form they become extended in tension, but return to their original shapeswhen the stress is removed. In raw rubber (i) a steady tensile force will cause separatemolecules to glide slowly past each other into new positions, so that when the force isremoved some plastic deformation remains, although the elastic deformation has dis-appeared. By 'vulcanising' the raw rubber (ii) the chain molecules are bonded together sothat no permanent plastic deformation can occur and only elastic deformation is possible.

(i) (ii)

FINAL POSITIONSOF MOLECULES

UNDER THEACTION OF

STRESS

STRESSSTRESS

INITIAL POSITIONSOF MOLECULES BOND

VULCANIZEDRUBBER

RAW~R"tiBBER

1.76 The Metallic Bond In most pure metals, atoms possess insufficientvalence electrons to be able to form covalent bonds with each other. Onthe other hand any metallic ions which may be formed in a pure metalwill carry like positive charges and so tend to repel each other so thatelectrovalent bonding is impossible. Yet we know that metals are crystal-line in the solid state (3.10). How then is this situation achieved?

The explanation generally offered is that the valence electrons of eachatom are donated to a common 'cloud' which is shared by all atoms present(Fig. 1.8). Thus, whilst the positively-charged ions which result, repel eachother so that they arrange themselves in a regular pattern, they are heldin these equilibrium positions by their mutual attraction for the negativelycharged electron cloud which permeates them. Individual electrons nolonger 'belong' to any particular atom but are the common property of allatoms present.

Fig. 1.8 Diagrammatic representation of the metallic bond.

A more detailed knowledge of the structure of the atom would indicatethat the situation is not nearly so simple that metallic bonding can beexplained in terms of this 'electron cloud' concept. However, for our pre-sent purposes it will be sufficient if we accept the results of this simpli-fied interpretation, since it enables us to explain many characteristicallymetallic properties.

Since valence electrons in the common 'cloud' are able to travel freelyamong the positive ions this gives an explanation of the high electricalconductivity of metals, a current of electricity being nothing more than amovement of electrons in a particular direction. In a covalently-bondedcompound on the other hand valence electrons are held captive in thechemical bond. Consequently most of the organic compounds—polythene,PVC and nylon—are insulators whilst liquids such as alcohols, benzeneand oils are non-electrolytes.

The opaque lustre of metals is due to the reflection of light by free

ELECTRON'CLOUD*

POSITIVEIONS

electrons. A light wave striking the surface of a metal causes a free electronto vibrate and absorb all the energy of the wave, thus stopping trans-mission. The vibrating electron then re-emits the wave from the metalsurface giving rise to what we term 'reflection'.

The very important property of most metals in being able to undergoconsiderable plastic deformation is also due to the existence of the metallicbond. Under the action of shearing forces, layers of positive ions can bemade to slide over each other without drastically altering their relationshipwith the shared electron cloud.

Secondary Bonding Forces

1.80 Stable atoms—and molecules—always contain equal numbers ofprotons and electrons. Consequently they will be electrically neutral andcarry no resultant charge. Yet they must attract each other for how elsecan we explain the fact that all gases condense to form liquids and that allliquids either crystallise to form solids or else become so viscous that thestrong forces of attraction between molecules make them behave almostlike solids?

In short, whilst we have related the cohesion between metallic particlesto the metallic bond we have not yet attempted to explain the forces whichhold together covalently bonded molecules, or, for that matter, the singleatoms in noble gases which, although they contain no valence electrons,must attract each other in some way since they ultimately liquify andsolidify at very low temperatures. These weak secondary bonding forcesare often referred to as van der Waals' forces since it was this Dutchphysicist who first explained the deviations in the Gas Law (PV = RT) asbeing partly due to forces of attraction between molecules (or atoms in thecase of noble gases) within the gas.

Consider two atoms of a noble gas such as argon. If these two atoms arein close proximity and their electrons happen to be concentrated as in Fig.1.9, it is reasonable to suppose that mutual attraction will occur betweenthe positively charged nucleus of atom X and the negatively charged elec-trons of atom Y at the moment when the nucleus of X is 'unshielded'by its own electrons. This situation will be continually changing as thedistribution of electrons alters, but a resultant weak force of attractionexists.

Engineers will be familiar with the idea of 'centre of mass' in a solidbody. In a similar way we can imagine electrical charges 'resolved' to give'centres' of positive and negative charges respectively in a molecule. If

Fig. 1.9.

YX

Fig. 1.11 The strong dipole moment of the water molecule (i) resulting in strong attractionbetween neighbouring molecules.

these centres do not coincide (Fig. 1.10) then the molecule will have asmall dipole moment and will consequently attract (and be attracted by)other molecules with similar dipole moments.

In a molecule of water (Fig. 1.11 (i)) the two electrons which are contrib-uted to the covalent bonds by the two hydrogen atoms tend to be drawnto the vicinity of the larger oxygen atom. Thus the centre of negativecharge is shifted nearer to the oxygen atom leaving the positively-chargednuclei of the hydrogen atoms relatively 'exposed'. Consequently the watermolecule has a very strong dipole moment and therefore a relatively strongforce of attraction for its neighbours (Fig. 1.11 (ii)).

For this reason water has an abnormally high freezing point and boilingpoint as compared with other substances of similar molecular size. Forexample, methane melts at — 183°C and boils at — 162°C.

Particularly strong van der Waals' forces arise from the behaviour of thehydrogen atom in this way and are referred to as 'hydrogen bonds'.

When considered singly van der Waals' forces are very weak when com-pared with the forces acting within a single covalent bond. The combinedeffect, however, of van der Waals' forces acting at a large number ofpoints between two adjacent chain-like polymer molecules such as those ofpolythene (1.75) can be very considerable. It also explains why polymermaterials, though weaker than metals are highly plastic.

Isotopes

1.90 In the foregoing discussion of the mechanism of chemical combi-nation no mention was made of the part played by the neutron. In fact theneutron, carrying no resultant electrical charge, has no apparent effect on

Fig. 1.10 A molecule with a resultant dipole moment.

(i) (ii)

ordinary chemical properties which are mainly a function of the electronstructure of the atom. The principal role of the neutron is to increase theactual mass of an atom. Thus, the sodium atom with 12 neutrons in thenucleus, in addition to the 11 protons already mentioned, has a totalnuclear mass of 23. The 11 electrons present are negligible in mass whencompared with the massive protons and neutrons, so that the mass of thetotal atom is approximately 23 units, and it is from this value that therelative atomic mass is derived.

There are many instances, however, in which two atoms contain thesame number of protons but unequal numbers of neutrons. Clearly, sincethey have equal numbers of protons, they will also have equal numbers ofelectrons and, chemically, such atoms will be identical. Differing numbersof neutrons, however, in respective atoms will cause these atoms to haveunequal masses. An element possessing atoms which are chemically identi-cal but which are of different mass is said to be isotopic and the differentgroups are known as isotopes.

1.91 Two such isotopes occur in the element chlorine. The chemicalproperties of these isotopes are identical because in each case an atom willcontain 17 protons and 17 electrons. Only the relative masses of each atomwill be different, since the nucleus of isotope II contains two more neutronsthan that of isotope I (Fig. 1.12). Since there are about three times asmany atoms of isotope I (usually written 35Cl) as of isotope II (written37Cl) the relative atomic mass of chlorine 'averages out' at 35.45.

Fig. 1.12 The particle 'make-up' of the two isotopes of chlorine.

1.92 There are three isotopes of hydrogen of atomic masses one, twoand three respectively (Fig. 1.13). The 'ordinary' hydrogen atom (nowcalled 'protium') contains a single proton in its nucleus—and no neutrons.Its atomic mass is therefore one. However, approximately one hydrogenatom in every 6900 also contains a neutron in its nucleus and since theproton and neutron are roughly equal in mass then the atomic mass of the

ISOTOPE I ISOTOPE LI

17 PROTONS18 NEUTRONS

17 PROTONS2O NEUTRONS

NUCLEUS17 PROTONSM^NEUTRONS35_ ..TOTAL WEICHT

ELECTRONS NUCLEUS17 PROTONS2Q NEUTRONS37 = TOTAL WEIGHT

Fig. 1.13 The three isotopes of hydrogen.Each isotope has the same electron/proton make-up so that the simple chemical propertiesof all three will be similar.

atom will be two. This atom, often known as 'heavy hydrogen', fH, is alsogiven the special name deuterium and is the basis of the 'hydrogen bon breaction'. A third, radioactive, isotope of hydrogen, called tritium, contai stwo neutrons along with the proton in the nucleus and so has an atomicmass of three, ie iH. It must be emphasised that deuterium and tritiumare not different elements from hydrogen but only isotopes of that element.It is perhaps fortunate that separate isotopes of other elements are notgiven special names in this manner or confusion would be rife. The term'isotope' tends to be associated in modern technology with the release ofnuclear energy by suitable elements. The properties of the isotopes ofuranium in this connection are dealt with later (18.75).

Exercises1. If the valence of aluminium is three, write down the chemical formula of its

oxide (1.31)2. What mass of metallic copper would be deposited by the electrolysis of 1Og of

copper sulphate (CUSO4) in water solution? (1.33)3. Calculate the mass of iron obtained by reducing 10 tonnes of the ore hematite,

assuming that hematite as mined contains 70% of the oxide Fe2O3. (1.33)4. Why must aluminium be obtained from its ore by electrolysis instead of by the

more usual process of reduction by carbon? (1.44)5. In the chemical reaction: Fe3O4 + C = 3FeO + CO, has the Fe3O4 been

oxidised or reduced? (1.44)6. Complete (and balance) the chemical equation:

HNO3 + MgO = ? (1.51)Nitric Magnesiumacid oxide

7. The melting points of Li, Na, K and Rb are 179, 97, 62 and 39°C respectively.Estimate graphically the melting point of Cs. (1.69 and Fig. 1.2)

8. Without reference to any tables sketch (i) the electron structure of the elementwhich has an atomic number of 14 (1.67); (ii) the electron structure of themagnesium ion. (1.71)

9. Show how far modern theory is successful in explaining not only many of the

electronprotonneutron

TritiumDeuteriumOrdinaryHydrogen

mechanical properties of a metal but also that it is a conductor of electricity.(1.76).

10. Explain why water has abnormally high freezing point and boiling point ascompared with substances of similar molecular mass such as ammonia. (1.80)

11. The metal copper (relative atomic mass—63.55) exists as two isotopes. 69.2%by mass of the metal consists of the isotope with a mass number 63. What isthe likely mass number of the other isotope? (1.91)

BibliographyBrown, G. L, A New Guide to Modern Valency Theory, Longman, 1971.Cooper, D. G., Chemical Periodicity, John Murray, 1974.Companion, A. L., Chemical Bonding, McGraw-Hill, 1979.Cox, P. A., The Elements, their origin, abundance and distribution, Oxford Science

Publications, 1989.Hume-Rothery, W. and Coles, B. R., Atomic Theory for Students of Metallurgy,

Institute of Metals, 1969.Underwood, D. M. and Webster, D. E., Chemistry, Edward Arnold, 1985.Wilson, J. G. and Newall, A. B., General and Inorganic Chemistry, Cambridge

University Press, 1971.