Microscopic World I

Unit 5 Atomic structure

Unit 6 The periodic table

Unit 7 Ionic and metallic bonds

Unit 8 Covalent bonds

Unit 9 Relating the properties of substances to structures and bonding

Topic 2

KeyC o ncepts

Covalent bonds• Covalent bonds andmolecules• Chemical formulaeof covalent

compounds• Predicting the formationof ionic and

covalent compounds• Dative covalentbonding• Relative molecularmass and formula

mass

Relating the properties of substances to structures and

bonding• Giant ionic structures• Giant covalent structures• Simple molecular structures• Giantmetallic structures

Microscopic World I

Atomic structure• Atomic number andmassnumber• Isotopes• Relative atomicmass• Electronic arrangement

Ionic and metallic bonds• Ionic bonds• Names and chemical formulaeof ionic

compounds• Metallic bonds inmetals

The periodic table• Periodic table, groups andperiods• Alkali metals• Alkaline earthmetals• Halogens• Noble gases

Topic 2� Microscopic World I �Unit 5 Atomic structure

5.1 – 5.11

Summary

1 All elements aremadeof atoms.

2 Chemistsuse symbols to represent elements.

3 Atroomtemperatureandpressure,elementsexistindifferentstates(solid,liquidorgas).

4 Elements can be classified into three main groups — metals, metalloids and non-metals.

Elements

metalloidsmetals non-metals

5 An atom consists of three types of subatomic particles: protons, neutrons andelectrons.

The nucleus contains protons and neutrons. Electrons move around the nucleusin shells.

neutron

nucleus

proton

electrons move around the nucleus in shells

6 Atomicnumberof an element =numberofprotons in an atomof that element =numberof electrons in aneutral atomof that element

7 Massnumber=numberofprotons+numberofneutrons

mass number

atomic number

symbol of an atom

8 Isotopesaredifferentatomsofanelementwhichhavethesamenumberofprotonsbut a different number of neutrons. For example, chlorine has two isotopes: Cl-35andCl-37.

9 The relative isotopicmassof aparticular isotopeof anelement is the relativemassof one atomof that isotopeon the 12C=12.00 scale.

5.1 What is an elementmadeof?

5.2 Symbols for elements

5.3 States of elements

5.4 How to classify elements?

5.5 Basic structureof an atom

5.6 Atomic number

5.7 Mass number

5.8 Isotopes

5.9 Relative massesof atoms and the carbon-12 scale

5.10 The arrangementof electrons in atoms

5.11 Electrons andorbitals

Unit 5 Atomic structure

Topic 2� Microscopic World I �Unit 5 Atomic structure

10Therelativeatomicmassofanelementistheweightedaveragerelativeisotopicmassof all thenaturallyoccurring isotopesof that elementon the 12C=12.00 scale.

11The way in which electrons are arranged in an atom is called its electronicarrangement.

nucleus

1st shell can hold a maximum of 2 electrons [2(1)2= 2]2nd shell can hold a maximum of 8 electrons [2(2)2 = 8]3rd shell can hold a maximum of 18 electrons [2(3)2 = 18]4th shell can hold a maximum of 32 electrons [2(4)2 = 32]

12An orbital is the region in which there is a high probability of finding anelectron.

Answer

87.6= 86 xp + 87 x (100 – p – 76.5) + 88 x 76.5100

(1)

p= 16.5 (1) q= 100 – 16.5 – 76.5 = 7.0 (1)

ExamtipsExamtipsExamtipsExamtips ♦ Questions often ask students to describe the number of subatomicparticles in an ion.

e.g.

The doubly charged cation formed from 5525Mn contains 23 electrons

and30neutrons.

♦ Questionsoftenaskstudentstoidentifyelementsbasedontheiratomicnumbers.

♦ Questionsoftengivethenumberofsubatomicparticlesinsomeatomsand ask students to identify their relationship (e.g. they are isotopes).

♦ Isotopesaredifferentatomsof thesameelement.Chemicalpropertiesof isotopesare the same. IsotopesCANNOTbe separatedby chemicalmeans.

e.g.

10BCl3and11BCl3arecompoundsformedfromthetwoisotopesofboronwith chlorine. Suppose 10BCl3 reacts with water to give white fumes.11BCl3would react to givewhite fumes aswell.

Example

A sample of strontium (Sr) consists of three isotopes 86Sr, 87Sr and 88Sr. The relativeabundanceof each isotope is shown in the tablebelow:

Isotope 86Sr 87Sr 88Sr

Relative abundance (%) p q 76.5

Therelativeatomicmassofthesampleofstrontiumis87.6.Calculatethevaluesofp and q. (3marks)

➤Isotopes of an element do NOThave the samemass.

➤Questions may give the relative abundance of the isotopes of an elementand ask students to calculate the relative atomicmass of the element.

RemarksRemarks*

Topic 2� Microscopic World I �Unit 6 The periodic table

6.1 How to group elements together?

6.2 The periodic table

6.3 Patterns across theperiodic table

6.4 Group I elements—alkalimetals

6.5 Group II elements—alkaline earthmetals

6.6 Group VII elements—halogens

6.7 Group 0 elements—noble gases

6.8 Predicting the chemicalproperties ofunfamiliar elements

6.9 From atoms to ions

6.10 Predicting the chargeon an ion

The periodic tableUnit 6 6.1 – 6.10

Summary

1 In the periodic table, all the elements are arranged in order of increasing atomicnumber.

2 The vertical columns in the periodic table are called groups. Groups are numberedfrom I toVII, followedbyGroup0 (orGroupVIII).

Groupnumberof an element=numberofoutermostshellelectrons inanatomofthe element

3 Thehorizontal rows in theperiodic table are calledperiods.

Periodnumberof an element= number of occupied electron shells in an atom ofthe element

4 Across a period in the periodic table, the elements change from metals throughmetalloids tonon-metals.

5 a) Elements in the samegrouphave the samenumberofoutermost shell electronsin their atoms and thus theyhave similar chemicalproperties.

b)Thereisusuallyagradualchangeinthepropertiesofelementsaswemovedowna group.

6 Group I elements—alkalimetals

a) They all have relatively low melting and boiling points when compared withothermetals.

b)They are all soft and canbe cutwith a knife.

c) They allhave lowdensities— lithium, sodiumandpotassium float onwater.

d)They are all reactive metals and must be stored in paraffin oil to prevent themfrom reactingwith air.

e) They all react vigorously with water to give hydrogen gas and an alkalinesolution.

f) They all reactwithnon-metals to formcompounds called salts.

g) The reactivityof these elements increases aswemovedown the group.

7 Group II elements—alkaline earthmetals

a) They all have relatively low melting and boiling points when compared withothermetals (exceptGroup Imetals).

b)They allhave lowdensities.

c) Theyareallreactivemetalsandreactreadilywithdilutehydrochloricacidtogivehydrogengas.

d)They all reactwithnon-metals to formcompounds called salts.

e) GroupIIelementsarelessreactivethanGroupIelements.Thereactivityincreasesaswemovedown the group.

Topic 210 Microscopic World I 11Unit 6 The periodic table

8 GroupVII elements—halogens

a) They are all poisonous and smelly.

b)They are allnon-metals.

c) They all reactwithmetals to formcompounds called salts.

d)The reactivityof these elementsdecreases aswemovedown the group.

9 The following diagram shows the trends of some physical properties and reactivityofGroups I, II andVII elements.

Fr Ra

Cs Ba

Rb Sr

K Ca

Na Mg

Li Be F

Cl

Br

I

• melting and boiling points increasing

• reactivity decreasing

• melting and boiling points decreasing

• reactivity increasing

• melting and boiling points increasing

• reactivity decreasing

10Group0 elements—noble gases

a) They are all colourless gases at room temperature andpressure.

b)They allhavevery lowmelting andboilingpoints.

c) They are all veryunreactive.

11Theoctetrulesuggeststhatatomsbecomestablebyhavingeightelectrons(anoctetstructure) in theiroutermost shells (or twoelectrons,aduplet structure, in thecaseof some smaller atoms).

12Atoms can obtain the stable electronic arrangements of atoms of noble gases bygainingor losing electrons.

13a) Positive charge(s) on an ion formed from the atomof ametal = groupnumberof themetal

b)Negative charge(s) on an ion formed from the atomof anon-metal = 8 – groupnumberof thenon-metal

GroupI

Group II

Group III

Group IV

Group V

Group VI

Group VII

Group 0

Period2 Li+ Be2+ N3– O2– F–

Period3 Na+ Mg2+ Al3+ S2– Cl–

Period4 K+ Ca2+ Br–

ExamtipsExamtipsExamtipsExamtips ♦ In the periodic table the elements are arranged in increasing order ofatomic number, NOTneutron number or relative atomicmass.

4 ✘ ✘

♦ Questions often ask about trends when going across a period of theperiodic table.

e.g.

Going across the second period of the periodic table (from lithium tofluorine),

– the elements show a gradual change from having metallic propertyto having non-metallic property;

– the elements have the same number of occupied electron shells intheir atoms;

– the atomic size of elements decreases;

– the melting points and boiling points of elements do NOT varygradually across a period.

♦ Theatomicradiusoftheelementsincreasesdownagroupaselectronsareaddedtosuccessiveelectronshellsandthusfurtherawayfromthenucleus.

♦ The melting point, boiling point, density and hardness of Group Ielements are all verymuch lower than the average values formetals.

♦ Below are some safety precautions for handling Group I elements inthe laboratory:

– wear safety glasses;

– use forceps;

– use a safety screen.

♦ Questions may ask students to deduce the atomic number of anelement from the known atomic number of another element in thesamegroup.

e.g.

XandYarealkalineearthmetals. TheatomicnumberofX isx. Thenthe atomic number ofY could be x + 18.

♦ When Barium (a Group II element) is added to a trough of watercontaining phenolphthalein,

– thepieceof barium sinks in thewater;

– a colourless gas is liberated;

– an alkaline solution forms and thephenolphthalein turns pink.

♦ The reactivity of elements in Group I / II increases down thegroup.

♦ Questions may ask students to comparepotassium and calcium.

– The reducing power of potassium is stronger than that of calcium.

– Thehardness of calcium is higher than that of potassium.

– Thedensity of calcium is higher than that of potassium.

Topic 212 Microscopic World I 13Unit 6 The periodic table

➤Questions often ask about thehalogens.

– Bromineistheonlynon-metalwhichisaliquidatroomtemperatureandpressure.

– Iodine vapour is purple in colour.

– The boiling point of chlorine, bromine and iodine increases down thegroup.

– The reactivity of fluorine, chlorine and bromine decreases with relativeatomicmass.

– Fluorine, chlorine and bromine all reactwith sodium sulphite solution.

➤Astatine is a halogen below iodine.

– It is soluble in heptane.

– Theoxidizing power of chlorine is stronger than that of astatine.

➤Questions often ask about phosphorus 3115P.

– Phosphorus belongs toGroup V and Period 3of theperiodic table.

– It is a solid at room temperature and pressure.

– Its atom contains 16 neutrons.

– It is a non-metal. It forms covalent compounds with non-metals.

– It forms ionic compounds withmetals.

RemarksRemarks*

♦ Notallatomsofnoblegaseshaveanoctet structure in theoutermostshell.Atomsof helium have a duplet structure.

♦ Questionsmaygive thechargecarriedbya stable ionofanunknownelement and ask thepossible atomic number of the element.

e.g.

An element forms a stable X2+ ion. The atomic number of X may be20or 38.

♦ Questions often ask about the conversion of an atom of a non-metalto form an anion.

e.g.

When an iodine atom is converted to an iodide ion,

– a reduction is involved;

– the iodide ion has the same number of occupied electron shells asthe iodine atom;

– the iodide ion and the iodine atom have the same number ofprotons.

♦ Questions may describe the number of subatomic particles in the ionofanunknownelementandaskstudentstodeduceinformationaboutthe element / its compound (e.g. the group or period to which theelement belongs) or vice versa.

e.g.

Suppose X2+ ion has an electronic arrangement 2,8,8. Thus, an atomofXhas an electronic arrangement 2,8,8,2.X is calcium.

The question may ask students to deduce information about thecarbonate ofX:

– it is awhite solid;

– it decomposes onheating;

– it produces a brick-red flame in flame test.

iii)Explain, in terms of bonging and structure, why the boiling point of element Zishigher than thatof elementX. (3marks)

b)DrawanelectrondiagramofacompoundformedbetweenelementsXandY,showingelectrons in theoutermost shells only. (1mark)

Answer

a) i) Halogens (1)

ii)p = 18 (0.5)

q = 7 (0.5)

iii)The boiling point of a substance depends on the strength of its intermolecularattractions. (1)

The intermolecularattractionsbetweenmoleculesofGroupVIIelementsarevanderWaals’ forces. (1)

A molecule of element Z has more electrons than a molecule of element X. /ElementZhas a biggermolecular size than elementX. (1)

Thus,thevanderWaals’forcesbetweenmoleculesofelementZarestrongerthanthosebetweenmolecules of elementX.

b)

(1)

X Y X

X

Example

The electronic arrangements of the atoms of three elements are shown in the tablebelow:

Element Electronic arrangement of atom

X 2,7

Y 2,8,5

Z 2,8,p,q

a) ElementsX andZbelong to the samegroupof theperiodic table.

i) Name the group towhich theybelong. (1mark)

ii)The atomicnumberof elementZ is 35.What are the valuesof p and q? (1mark)

Topic 21� Microscopic World I 1�Unit 7 Ionic and metallic bonds

7.1 Conductors, electrolytes andnon-conductors

7.2 Evidence of ions fromelectrolysis ofmolten lead(II) bromide

7.3 Chemical bonds

7.4 Ionic bonds

7.5 Compounds containingpolyatomic ions

7.6 Names of ions

7.7 Naming ionic compounds

7.8 Colours of ionic compounds

7.9 Chemical formulaeof ionic compounds

7.10 Metallic bonds inmetals

Ionic and metallic bondsUnit 7 7.1 – 7.10

Summary

1 Substances canbe classified as conductors, electrolytes andnon-conductors.

conductors electrolytes non-conductors

metals non-metals

compounds made up of metals and non-metals

compounds made up ofnon-metals

2 An ionic bond is the strong electrostatic forces of attraction between oppositelycharged ions.

a) An ionic bond is formed when one or more electrons are transferred from oneatom (or groupof atoms) to another.

b)When a metal and a non-metal combine to form an ionic compound, atoms ofthemetal release electronswhile atomsof thenon-metal gain electrons.

3 The tablebelow shows electrondiagramsof some common ionic compounds.

Ionic compound Electron diagram

Sodiumchloride Cl

–

Na

+

Magnesium fluoride Mg2+

–

F

–

F

Lithiumoxide

2–

O

Li

+

Li

+

4 Apolyatomic ion is formed froma groupof atoms, insteadof a single atom.

Topic 21� Microscopic World I 1�Unit 7 Ionic and metallic bonds

5 a) Namingpositive ions:

i) If a metal forms only one kind of positive ion, the name of the ion is thesame as themetal.

ii)Some metals can form more than one kind of positive ion. For example, weuse thenameof copper(I) ion forCu+, and copper(II) ion forCu2+.

b)Namingnegative ions:

i) Simplenegative ionshavenames ending in ‘-ide’.

ii)Polyatomic ions containingoxygenhavenames ending in -ite or -ate.

iii)Thepolyatomicionwithlessoxygenisnamed-ite,andthatwithmoreoxygenisnamed -ate.

iv)Apolyatomicionformedfromanoxygenatomandahydrogenatomiscalledahydroxide ion.

6 When naming an ionic compound, name the positive ion first, followed by thenegative ion.

7 If an ionic compoundhas colour, the colourmay arise fromeither thenegativeorpositive ion, or even fromboth ions.

Ion Chemical formula Colour

Iron(II) Fe2+ pale green

Iron(III) Fe3+ yellow-brown

Copper(II) Cu2+ blueor green

Permanganate MnO4– purple

Dichromate Cr2O72– orange

Chromium(III) Cr3+ green

Nickel(II) Ni2+ green

Manganese(II) Mn2+ verypalepink (or colourless)

8 The chemical formula of an ionic compound shows the types of ions present andthe ratioof one typeof ion to theother.

9 A metallic bond is a type of bond in which positive metal ions are held togetherby a ‘sea’ ofmobile electrons.

The followingdiagram shows themetallic bonding in apieceofmetal.

Example

X,YandZ are threedifferent elements.X2+ ionandY3– ionandneonatomhave thesame electronic arrangement.

a) What are elementsX andY? (1mark)

b)DrawanelectrondiagramofthecompoundformedfromelementsXandY,showingelectrons in theoutermost shells only. (1mark)

c) ElementZbelongstothesameperiodaselementXandthesamegroupaselementY in theperiodic table.What is elementZ? (1mark)

Answer

a) ElementX ismagnesium. (0.5)

ElementY isnitrogen. (0.5)

ExamtipsExamtipsExamtipsExamtips ♦ A piece of freshly cut lithium turns dull when placed in air. Lithiumoxide is formed.

♦ It is WRONG to draw the electron diagram of lithium oxide in thefollowing way:

+

Li

2–

2

O

✘

♦ Studentsshouldbeabletodescribethestructureandbondinginmetalwith thehelp of a diagram.

Notice that cations and delocalized electrons are involved in metallicbonding.

♦ The binding forces in both metallic bond and ionic bond are non-directional.

Metallic bond differs from covalent bond in that metal atoms do notformseparatemolecules.However,thaatomiccoresareboundtogetherbybeingattractedtotheelectronsbetweentheminbothmetallicbondand covalent bond.

Topic 21� Microscopic World I 1�Unit 8 Covalent bonds

8.1 Covalent bonds

8.2 Covalent bonds innon-metallic elements

8.3 Covalent compounds

8.4 Writing chemical formulaeof covalent compounds

8.5 Predicting the formationof ionic and covalent compounds

8.6 Dative covalentbonding

8.7 Bonding inpolyatomic ions

8.8 Relative molecularmass and formulamass

Covalent bondsUnit 8

➤Questions often ask about the chemical formula and electron diagram ofmagnesiumnitride (Mg3N2).

RemarksRemarks*

b)

(1)

3–

N

3–

N

2+

Mg

2+

Mg

2+

Mg

c) Phosphorus (1)

Topic 220 Microscopic World I 21Unit 8 Covalent bonds

8.1 – 8.8

Summary

1 Acovalentbondis formedwhenoneormorepairsofoutermostshellelectronsaresharedbetween two atoms.

2 A covalent bond is the strong electrostatic forces of attraction between the sharedelectrons and the twopositively chargednuclei of thebonded atoms.

3 Atoms of non-metallic elements can join together to form groups calledmolecules.

4 The table below shows the electron diagrams and models of molecules of somecommonnon-metals.

Molecule Electron diagram Ball-and-stick model

Hydrogen

bond pair electrons

hydrogen molecule (H — H)

H H

H H

Chlorine Cl Cl

chlorine molecule (Cl — Cl)

lone pair electrons

Cl Cl

OxygenO

oxygen molecule (O = O)

O

O O

NitrogenN

nitrogen molecule (N ≡ N)

N

N N

5 Atoms of different non-metallic elements share electrons with each other to formcovalent compounds.

6 The table below shows the electron diagrams and models of molecules of somecommoncovalent compounds.

Molecule Electron diagram Ball-and-stick model

Hydrogen chloride

hydrogen chloride molecule (H — Cl)

H Cl

H Cl

Tetrachloromethane

Cl

Cl

ClCl C

tetrachloromethane molecule

Cll

( Cl — C — Cl )l

Cl

Cl C

Cl

Cl

Cl

WaterH HO

water molecule (H — O — H)

H O H

AmmoniaH

H HN

ammonia molecule (H — N — H)

lH

H N

H

H

CarbondioxideO C O

carbon dioxide molecule(O = C = O)

O C O

Topic 222 Microscopic World I 23Unit 8 Covalent bonds

7 a) Whenametal combineswith anon-metal, an ionic compound forms.

b)Whennon-metals combine, a covalent compound forms.

metal and non-metalcombine to form ionic compound

non-metalscombine to

form covalent compound

non-metallicelement

particlespresent

particlespresent

particlespresent

particlespresent

particlespresent

ions molecules atoms(a networkof covalent

bonds linking atoms)

molecules atoms(a networkof covalent

bonds linking atoms)

examples examples examples examples examples

magnesiumoxide,

sodiumchloride

eammonia,carbondioxide

silicondioxide

argon(exists as

monoatomicmolecules),chlorine,hydrogen

carbon,silicon

metal non-metal

8 A dative covalent bond is a covalent bond in which the bond pair electrons areprovidedby the same atom.

ThefollowingdiagramshowstheformationofadativecovalentbondinanNH3BF3molecule.

+H N H NB

F

F

F B

F

F

H

H

F

H

H

9 Insubstancescontainingpolyatomic ions, thecationsandanionsareheld togetherby ionicbonding,but eachpolyatomicanion is agroupof atomsheld togetherbycovalentbonding.

Thefollowingdiagramshowstheformationofadativecovalentbondinanammoniumion.

+H H+ H N H

H

H

H

H

+

N

10Relativemolecularmassofanelementorcompound=sumofrelativeatomicmassesof all atomspresent inonemoleculeof the elementor compound

11Formula mass of an ionic compound = sum of the relative atomic masses of allatomsmakingupone formulaunit of the compound

ExamtipsExamtipsExamtipsExamtips ♦ Questions often ask why noble gases exist as monoatomic moleculesorwhy they are unreactive.

♦ Chlorine exists as diatomic molecules, NOT ‘bimolecular’. ✘

♦ Questionsmay ask students todrawelectrondiagramsof compoundsformed from twohalogens.

e.g.

F Cl

(Only electrons in theoutermost shells are shown.)

♦ Hydrogenperoxide (H2O2) is anoxide of hydrogen.

H O O H

(Only electrons in theoutermost shells are shown.)

♦ Notice that both ionic and covalent bonds exist in ammoniumnitrate.

Example

X,YandZarethreeconsecutiveelements intheperiodictable,withthesumoftheiratomicnumbers equal to54.X forms a stable anionX–whileZ forms a stable cationZ+.

a) What are elementsX,Y andZ? (1mark)

b)PisacompoundformedfromelementXandoxygen.DrawanelectrondiagramofP, showing electrons in theoutermost shells only. (1mark)

c) Q is a compound formed for element Z and oxygen. Draw an electron diagram ofQ, showing electrons in theoutermost shells only. (1mark)

d)Compare themeltingpoints of compoundsP andQ. Explainyour answer. (3marks)

Answer

a) X: chlorine

Y: argon (1)

Z: potassium

b)

(1)

Cl O Cl

Topic 22� Microscopic World I 2�Unit 9 Relating the properties of substances to structures and bonding

c)

(1)

2–

O

+

K

+

K

d)Themeltingpointof compoundQ ishigher than thatof compoundP. (1)

To melt compound Q, a lot of heat is needed to overcome the strong ionic bondsbetween the ions. (1)

TheattractiveforcesbetweenthemoleculesofcompoundPareweakvanderWaals’forces. Littleheat isneeded to separate themolecules. (1)

➤Questions often ask students to explain the difference between meltingpoints of a substance with a giant ionic structure and a substance with asimplemolecular structure.

RemarksRemarks*

9.1 A look at the structuresof some substances

9.2 Giant ionic structures

9.3 Properties of ionic compounds

9.4 Giant covalent structures

9.5 Properties of substanceswith giant covalent structures

9.6 Applications of graphite

9.7 Simple molecular structures

9.8 Properties of substanceswith simplemolecular structures

9.9 Giant metallic structures

9.10 Properties ofmetals

9.11 Predicting properties of substances from their structures

9.12 Predicting structuresof substances from their properties

Relating the properties of substances to structures and bondingUnit 9

Topic 22� Microscopic World I 2�Unit 9 Relating the properties of substances to structures and bonding

9.1 – 9.12

Summary

1 The following table summarizes theproperties of substanceswith the four typesofstructures: giant ionic, giant covalent, simplemolecular andgiantmetallic.

Type of structure

Giant ionic Giant covalent Simple molecular Giant metallic

Where this type of

structure is found

compoundsformedbetweenmetals andnon-metals

Group IVelements andsomeof theircompounds

somenon-metallicelementsand somecompoundsformedbetweennon-metals

metals

Examples

sodiumchloride,magnesiumoxide

diamond,graphite, silicondioxide

hydrogen,iodine,ammonia, dryice

aluminium,copper,magnesium

Particles present

ions atoms smallmolecules

positive ionssurroundedby a ‘sea’ ofelectrons

example:NaCl

example:diamond

example:dry ice

example:magnesium

Bonding strong ionicbonds

atoms joinedbystrong covalentbonds

strong covalentbondswithinthemolecules;weakvanderWaals’ forceshold separatemoleculestogether

strongmetallicbonds

Common state(s) at room

temperature and

pressure

solid solid solid, liquidor gas solid

Hardness hardbutbrittle

veryhard (incaseof three-dimensionalnetworkstructure)

solidsusuallysoft

hard,malleableandductile

Melting point and

boiling point

high veryhigh low moderatelyhigh tohigh

Type of structure

Giant ionic Giant covalent Simple molecular Giant metallic

Solubility in water and non-aqueous solvents

usually solubleinwater butinsoluble innon-aqueoussolvents

insoluble inwater andnon-aqueoussolvents

usuallyinsoluble inwater butsoluble innon-aqueoussolvents

usuallyinsoluble inwater* andnon-aqueoussolvents

Electrical conductivity

goodconductorswhenmoltenor in aqueoussolution

non-conductors(exceptgraphite)

non-conductors goodconductors

* Some metals reactwithwater to givehydrogen.

2 Allotropes are two (or more) forms of the same element in which the atoms ormolecules are arranged indifferentways.

3 From the group numbers of elements that make up a compound, we can tell thetype of bonding present and the structure of the compound. We can then predicttheproperties of the compound from its structure.

4 Information such as state (at room temperature and pressure), melting point andelectrical conductivityof a substance canhelpus topredict its structure.

ExamtipsExamtipsExamtipsExamtips ♦ Students should be able to give themeaning of the term ‘allotrope’.

♦ Silicon and carbon belong to the same group. They have similarstructures.

The types of bonding and structure in silicon are covalent bond andgiant covalent network, but NOT ‘giant covalent bonds’.

✘

♦ Although carbon and silicon belong to the same group, they formoxideswithdifferentstructures.Inquartz,attractionsbetweentheatomsare covalent bonds. In carbon dioxide, attractive forces between themolecules areweak vanderWaals’ forces.

♦ When considering substances with simple molecular structures (e.g.carbon dioxide and iodine), it is important to distinguish between thecovalentbondsholdingatomswithin themoleculesand theweakvanderWaals’ forces between themolecules.

♦ Questions often ask about the structure of iodine.

♦ When iodine sublimes, it absorbs energy as energy is required toovercome the attractive forces between the iodine molecules.

♦ Moltensulphurcontainsmobilesulphurmolecules.MoltensulphurdoesNOT conduct electricity.

Topic 22� Microscopic World I 2�Unit 9 Relating the properties of substances to structures and bonding

Example

a) Drawa three-dimensionaldiagram for the structureof each solid substancebelow.

i) Diamond

ii) Graphite

iii) Sodiumchloride (3marks)

b)With reference to the bonding and structure of diamond, explain why diamond issohard. (2marks)

c) With reference to thebonding and structureof graphite, explainwhy

i) graphiteusedtomakepencilcorescanbeeasilydetachedto formmarkingsonpaper; (2marks)

ii) graphite canbeused tomake electrodes. (2marks)

d)Explainwhether solid sodiumchloride is an electrical conductor. (2marks)

Answer

a) i)

(1)key:

carbon atom

ii)

(1)

key:

van der Waals’ forces

carbon atom

♦ Questions may give some information of an unknown substance andask students to deduce thepossible structure(s) of the substance.

e.g.

AsubstancehasahighmeltingpointanddoesNOTconductelectricitywhen in solid state. Itmaybe

– a compound withgiant molecules;

– an elementwith a giant covalent structure; or

– a compound with a giant ionic structure.

iii)

(1)

key:

chloride ion

sodium ion

b)Diamondhas a giant covalent structure. (1)

Each atom is bonded to other atoms by strong covalent bonds. Relative motion ofthe atoms is restricted. (1)

c) i) There are weak van der Waals’ forces between the layers of carbon atoms ingraphite. (1)

When graphite is pressed onto a piece of paper, the layers can slide over eachother anddetach to formmarkingsonpaper. (1)

ii)In graphite, each carbon atom uses three electrons in forming covalent bondswith threeother carbonatoms. (1)

Theremainingoutermostshellelectronofeachcarbonatomisdelocalizedbetweenthe layers of carbonatoms. (1)

Hence graphite can conduct electricity.

d)Solid sodium chloride contains ions. The ions are held together by strong ionicbonds.

The ions arenotmobile. (1)

Hence solid sodiumchloride cannot conduct electricity. (1)

➤Questionsoftenaskabouttheallotropesofcarbon:diamond,graphiteandbuckminsterfullerene.

➤Studentsshouldbeabletodrawclearlythethree-dimensionalstructuresofdiamond and graphite.

➤Questionsoftenaskstudentstoexplainthepropertiesofgraphite/diamond/ buckminsterfullerene based on their bonding and structures.

➤Questions often ask students to explain why an ionic compound is aninsulator of electricity in the solid state, but it conducts electricity in themolten state.

Noticethatamoltenioniccompoundconductselectricityduetothepresenceofmobile ions, NOTdue to themovement of delocalized electrons.

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