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4.1 atomic structure and the periodic table Name:Class:

Author: J Tuckerman

Date:

Time: 60 minutes

Marks: 60

Key Content from Specifications

Atoms, Elements and Compounds

All substances are made of atoms. An atom is the smallest part of an element that can exist. Atoms of each element are represented by a chemical symbol, e.g. O represents an atom of

oxygen, Na represents an atom of sodium. There are about 100 different elements. Elements are shown in the periodic table. Compounds are formed from elements by chemical reactions. Chemical reactions always

involve the formation of one or more new substances, and often involve a detectable energy change. Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed. Compounds can only be separated into elements by chemical reactions.

Chemical reactions can be represented by word equations or equations using symbols and formulae.

Mixtures

A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged.

Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. These physical processes do not involve chemical reactions and no new substances are made.

The development of the model of the atom (common content with Physics)

New experimental evidence may lead to a scientific model being changed or replaced. Before the discovery of the electron, atoms were thought to be tiny spheres that could not

be divided. The discovery of the electron led to the plum pudding model of the atom. The plum pudding

model suggested that the atom is a ball of positive charge with negative electrons embedded in it.

The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model.

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.

Relative electrical charges of subatomic particles

The relative electrical charges of the particles in atoms are:

In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.

The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

Size and mass of atoms

Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m). The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m). Almost all of the mass of an atom is in the nucleus.

The relative masses of protons, neutrons and electrons are:

The sum of the protons and neutrons in an atom is its mass number. Atoms of the same element can have different numbers of neutrons; these atoms are called

isotopes of that element. Atoms can be represented as shown in this example:

Relative atomic mass

The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.

Electronic structure

The electrons in an atom occupy the lowest available energy levels (innermost available shells). The electronic structure of an atom can be represented by numbers or by a diagram. For example, the electronic structure of sodium is 2,8,1 or showing two electrons in the lowest energy level, eight in the second energy level and one in the third energy level.

The periodic table

The elements in the periodic table are arranged in order of atomic (proton) number and so that elements with similar properties are in columns, known as groups. The table is called a periodic table because similar properties occur at regular intervals.

Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties.

Development of the periodic table

Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.

The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.

Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.

Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.

Metals and non-metals

Elements that react to form positive ions are metals. Elements that do not form positive ions are non-metals. The majority of elements are metals. Metals are found to the left and towards the bottom of

the periodic table. Non-metals are found towards the right and top of the periodic table.

Group 0

The elements in Group 0 of the periodic table are called the noble gases. They are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons. The noble gases have eight electrons in their outer shell, except for helium, which has only two electrons.

The boiling points of the noble gases increase with increasing relative atomic mass (going down the group).

Group 1

The elements in Group 1 of the periodic table are known as the alkali metals and have characteristic properties because of the single (1) electron in their outer shell.

In Group 1, the reactivity of the elements increases going down the group.

Group 7

The elements in Group 7 of the periodic table are known as the halogens and have similar reactions because they all have seven (7) electrons in their outer shell. The halogens are non-metals and consist of molecules made of pairs of atoms (diatomic).

In Group 7, the further down the group an element is the higher its relative molecular mass, melting point and boiling point.

In Group 7, the reactivity of the elements decreases going down the group. A more reactive halogen can displace a less reactive halogen from an aqueous solution of its

salt.

Properties of transition metals (chemistry only)

Comparison with Group 1 elements

The transition elements are metals with similar properties which are different from those of the elements in Group 1.

Typical properties

Many transition elements have ions with different charges, form coloured compounds and are useful as catalysts.

Practice Questions:

Q1. (a) Which sub-atomic particles are present in the nucleus of an atom?

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(b) There are two isotopes of the element chlorine:

Describe, in terms of sub-atomic particles, one similarity and one difference between atoms of the two isotopes of chlorine.

Similarity .........................................................................................................

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Difference .......................................................................................................

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(c) Chlorine reacts with hydrogen to produce hydrogen chloride.

(i) The table shows the values of some bond dissociation energies.

Bond H—H Cl—Cl H—Cl

Dissociation energyin kJ per mole

436 242 431

Use the values in the table to calculate the enthalpy change (∆H ) for the reaction.

H2(g) + Cl2(g) 2 HCl(g)

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Enthalpy change (∆H ) = ......................... kJ per mole(3)

(ii) Hydrogen also reacts with fluorine.

H2(g) + F2(g) 2 HF(g) ∆H = –538 kJ per mole

Draw an energy level diagram for this reaction.

Include on your diagram labels to show:

• the reactants and the products• the overall enthalpy change (∆H )• the activation energy.

(3)

(Total 10 marks)

Q2. Glass is made from silicon dioxide.

© Velirina/iStock/Thinkstock

(a) Silicon dioxide has a very high melting point.

Other substances are added to silicon dioxide to make glass. Glass melts at a lower temperature than silicon dioxide.

Suggest why.

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(b) Sodium oxide is one of the substances added to silicon dioxide to make glass.

(i) Sodium oxide contains Na+ ions and O2– ions.

Give the formula of sodium oxide.

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(ii) Sodium oxide is made by heating sodium metal in oxygen gas.

Complete the diagram to show the outer electrons in an oxygen molecule (O2).

(2)

(c) Glass can be coloured using tiny particles of gold. Gold is a metal.

Describe the structure of a metal.

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(Total 7 marks)

Q3. (a) Why do the elements in Group 1 of the Periodic Table have similar chemical properties?

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(b) Explain why the reactivity of the elements in Group 1 increases down the group.

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(Total 3 marks)

Q4. (a) The diagram shows the electronic structure of a particular element.

In a similar way, show the electronic structure of another element from the same group in the periodic table and name the element you select.

Name of element selected ..........................................................................................

(4)

(b) The element lithium gives a moderate reaction with cold water, releasing hydrogen and forming a solution of lithium hydroxide.

Describe how sodium is similar to and how it is different from lithium in its chemical reaction with cold water.

Explain any similarity or difference in terms of their atomic structure.

Similarity. ..................................................................................................................

Reason. ......................................................................................................................

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Difference. .................................................................................................................

Reason. ......................................................................................................................

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(Total 9 marks)

Q5.(a) Dmitri Mendeleev was one of the first chemists to classify the elements by arranging them in order of their atomic weights. His periodic table was published in 1869.

How did Mendeleev know that there must be undiscovered elements and how did he take this into account when he designed his periodic table?

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(b) By the early 20th century protons and electrons had been discovered.

Describe how knowledge of the numbers of protons and electrons in atoms allow chemists to place elements in their correct order and correct group.

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(c) The transition elements are a block of elements between Groups 2 and 3 of the periodic table.

(i) Transition elements have similar properties.

Explain why, in terms of electronic structure.

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(ii) There are no transition elements between the Group 2 element magnesium and the Group 3 element aluminium.

Give a reason why, in terms of electronic structure.

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(Total 8 marks)

Q6.This question is about metals.

Figure 1 shows the metals used to make pylons and the wires of overhead cables.

(a) An ore contains a metal compound.

A metal is extracted from its ore in three main stages, as shown in Figure 2.

Explain why Stage 2 needs to be done.

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(b) Cast iron from a blast furnace contains 96% iron and 4% carbon.

(i) Cast iron is not suitable for the manufacture of pylons.

Give one reason why.

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(ii) Most cast iron is converted into steel, as shown in Figure 3.

Describe how cast iron is converted into steel.

Use Figure 3 to help you to answer this question.

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(c) Aluminium and copper are good conductors of electricity.

(i) State one property that makes aluminium more suitable than copper for overhead cables.

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(ii) How can you tell that copper is a transition metal and aluminium is not a transition metal from the position of each metal in the periodic table?

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(iii) Copper can be extracted from solutions of copper salts by adding iron.

Explain why.

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(Total 10 marks)

Q7. The extract below was taken from a leaflet on the uses of platinum. One of the uses described was in making electrodes for spark plugs in car engines. The spark plug produces the spark which ignites the fuel in the engine.

Spark PlugsThe electrodes in a spark plug have to conduct electricity very well. Since they project into the combustion chamber of the engine, they must also be able to withstand extremely high temperatures in a very corrosive atmosphere.

Nickel-based plugs have been produced for many years. They only last a fairly short time. As the electrodes wear, combustion becomes less efficient and the petrol is not burnt completely.

Platinum and other precious metals can now be used in spark plugs. These last much longer and are more efficient. This can help to reduce air pollution.

The table below gives some information about platinum and nickel.

MELTINGPOINT(° C)

BOILINGPOINT(° C)

POSITION INREACTIVITY

SERIES

COST(£/kg)

nickel 1455 2920 Higher than gold 2.5

platinum 1769 4107 below gold 6110

(a) Compare nickel and platinum for use in making the electrodes in spark plugs.

A good answer should give advantages and disadvantages of each metal linking these to the properties of the metals. Marks will be given for the way in which you organise your answer.

You will need a sheet of lined paper.(8)

(b) (i) Describe the structure and bonding in metals.

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(ii) Explain why metals such as nickel and platinum are good conductors of electricity.

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(Total 13 marks)

M1.(a) neutron(s)answers can be in either order

1

proton(s)1

(b) same number (17) protons or same number electronsif candidate chooses to quote numbers, they must be correct

1

different numbers of neutrons (35Cl has 18 and 37Cl has 20)1

(c) (i) −184kJ / molcorrect answer with or without working gains 3 marksallow 2 marks for 184 kJ / molIf answer incorrect award up to 2 marks for any two of the steps below:• bonds broken: (436 + 242) = 678 (kJ)• bonds formed: (2 × 431) = 862 (kJ)• bonds broken − bonds formedallow ecf for arithmetical errors

3

(ii)

the reactants and the products at the correct levelignore labels on the axes

1

ΔH correctly labelledallow −538 if in correct place

1

Ea correctly labelledcorrectly labelled endothermic reaction gains max. 2 marks

1[10]

M2.(a) weaker bondsallow (other substances) react with the silicon dioxide

or

fewer bondsignore weaker / fewer forces

or

disruption to latticedo not accept reference to intermolecular forces / bonds

1

(b) (i) Na2Odo not accept brackets or charges in the formula

1

(ii)

electrons can be shown as dots, crosses, e or any combination

2 bonding pairsaccept 4 electrons within the overlap

1

2 lone pairs on each oxygenaccept 4 non-bonding electrons on each oxygen

1

(c) lattice / regular pattern / layers / giant structure / close-packed arrangement1

(of) positive ions or (of) atoms1

(with) delocalised / free electronsreference to incorrect particles or incorrect bonding or incorrect structure = max 2

1[7]

M3. (a) same number of electrons in outer shell/highest energy levelor they all form 1+ ionsIf number of electrons is given it must be correctAllow loss of same number of electrons to give full shellnot gain of electrons

for 1 mark1

(b) electrons are lost more readily/more easilyfurther from nucleus/bigger atom/more shielding/more energy levels/not just more electronsnot just reduced attraction

for 1 mark each2

[3]

M4. (a) 2.8.2. magnesium or 2.8.8.2. calciuminner shell (2)outer shell (2)intermediate shell/s correctelement named to match structure

each for 1 mark(Structure correct for element name but not in group2, award 2 marks)

4

(b) Similar hydrogen and/or an alkali/hydroxide producedfor 1 mark

Reasons: chemical reaction involves loss of an electron Na + Li have the same number of electrons in the outer shell (do not allow same group of p.table)

each for 1 mark

Different rate of reaction faster for sodium*for 1 mark

Reason: outer electron more easily lost from the sodium atom[* allow sodium hydroxide produced]

for 1 mark5

[9]

M5.(a) if placed consecutively, then elements would be in wrong group / have wrong properties

allow some elements didn’t fit pattern1

left gaps1

(b) (elements placed in) atomic / proton number order1

(elements in ) same group have same number of outer electrons1

any one from:

• number of protons = number of electrons

• reactions/(chemical) properties depend on the (outer) electrons

• number of shells gives the periodallow number of shells increases down the group

1

(c) (i) (transition elements usually) have same / similar number of outer / 4th shell electrons

allow 2 electrons in outer shell1

(because) inner (3rd ) shell / energy level is being filledignore shells overlap

1

(ii) 2 nd shell / energy level can (only) have maximum of 8 electronsaccept no d-orbitals

or2 nd shell / energy level cannot have 18 electrons1

[8]

M6.(a) The ore is not pure or contains impurities or the ore does not contain 100% of the metal compound

allow to concentrate the metal or metal compound1

rock / other compounds need to be removed / separated1

(b) (i) (cast iron is) brittleallow not strongignore weak

1

(ii) the oxygen reacts with carbonallow carbon burns in oxygen or is oxidised

1

reducing the percentage of carbon in the mixtureor producing carbon dioxide

1

(c) (i) aluminium has a low density1

(ii) (because copper) is in the central / middle (block of the periodic table)1

whereas aluminium is in Group 3 (of the periodic table)1

(iii) iron is more reactive (than copper)ignore cost

1

so copper is displaced / reduced1

[10]

M7. (a) 8 marks Particularly well structured answer with most points mentioned.

7-6 marks Well structured answer. The two metals will have beencompared rather than simply listing advantages/disadvantages. Mostof the advantages and disadvantages of each metal have been mentioned.

5-3 marks Some structure to the answer. An attempt to compare the metalsby giving some advantages and disadvantages.

2-1 marks Little structure or attempt to compare. Marks gained by listing a few advantages or disadvantages.

Advantages of Nickel:Relatively low cost which makes the sparking plugs cheaper to produce.Quite high melting point which is needed because the temperature in theengine is very high.Good conductor of electricity needed to carry electricity into combustionchamber to produce spark.

Disadvantages of Nickel:Subject to corrosion in engine which means they only last a short timebecause nickel is higher in reactivity than platinum.Idea that this leads to reduced efficiency, unburnt petrol and air pollution.

Advantages of Platinum:Less susceptible to corrosion (not corroded) because platinum is very low in reactivity.Idea that this improves efficiency and reduces pollution.-Higher melting point than nickel to withstand the high temperatures in the combustion chamber.Last a lot longer than nickel electrodes due to low reactivity. (Sensible extension here could be longer service intervals etc.)-Good conductor of electricity as for nickel.Extension here could be linked to the idea that the conductivity does not deteriorate as quickly as nickel.)

Disadvantages of Platinum:Cost which will make the sparking plug more expensive.A good candidate might justify cost by longer life, better fuel consumption and less pollution.

8

(b) (i) giant structure/lattice/regular arrangements of atomsany for 1 mark

of atoms/of ions (provided free electrons mentioned)either for 1 mark

delocalised or free electrons for 1 mark

3

(ii) electrons free/can move

for 1 mark each2

[13]

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