IGCSE Chemistry Paper 4 (Theory/Extended)

Topic 2: Atoms, Elements and Compounds -- Mock Exam 2

Duration 75 minutes

Total Marks 80

Questions 7

Time Remaining 75:00

Instructions

Answer ALL questions in the spaces provided.
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Question 1 -- Elements, Compounds and Mixtures

Total: 12 marks

A chemistry laboratory in Mumbai is testing a collection of substances. The technician has labelled each sample and needs to classify them correctly.

(a) [3]

Define each of the following terms:
(i) element
(ii) compound
(iii) mixture

Model Answer -- 1(a)

(i) An element is a substance that contains only one type of atom / cannot be broken down into simpler substances by chemical methods [1]

(ii) A compound is a substance formed when two or more elements are chemically combined / bonded together in a fixed ratio [1]

(iii) A mixture is two or more substances (elements or compounds) that are not chemically combined / can be separated by physical methods [1]

1 mark -- element contains only one type of atom or cannot be broken down chemically 1 mark -- compound is two or more elements chemically combined in fixed ratio 1 mark -- mixture is substances not chemically combined or separable by physical methods

(b) [4]

Classify each of the following substances as an element, compound or mixture. Give a reason for each answer.

(i) Copper wire
(ii) Sea water
(iii) Carbon dioxide gas
(iv) Bronze (an alloy of copper and tin)

Model Answer -- 1(b)

(i) Copper wire -- element; it contains only copper atoms / only one type of atom [1]

(ii) Sea water -- mixture; it contains water, dissolved salts, and other substances that are not chemically combined [1]

(iii) Carbon dioxide -- compound; it contains carbon and oxygen atoms chemically bonded together in a fixed ratio (CO₂) [1]

(iv) Bronze -- mixture; it contains copper and tin atoms mixed together but not chemically bonded / can vary in composition [1]

1 mark -- copper wire is an element because it contains only one type of atom 1 mark -- sea water is a mixture because substances are not chemically combined 1 mark -- carbon dioxide is a compound with atoms chemically bonded in fixed ratio 1 mark -- bronze is a mixture because metals are mixed not chemically bonded

A student in the laboratory has a mixture of iron filings and sulfur powder. Describe a physical method to separate the iron from the sulfur. Explain why this method works and state one observation the student would make.

Model Answer -- 1(c)

Pass a magnet over / through the mixture [1]

Iron is magnetic and is attracted to the magnet, but sulfur is not magnetic so it remains behind [1]

Observation: the grey/silver iron filings cling to the magnet and can be pulled away from the yellow sulfur powder [1]

1 mark -- use a magnet to separate the iron filings 1 mark -- iron is magnetic and attracted to magnet but sulfur is not 1 mark -- observation such as grey iron filings cling to magnet leaving yellow sulfur behind

(d) [2]

When iron filings and sulfur powder are heated strongly together, they react to form the compound iron sulfide (FeS). Iron sulfide has very different properties from both iron and sulfur.

Explain why the properties of iron sulfide are different from those of iron and sulfur.

Model Answer -- 1(d)

In a compound, the atoms of iron and sulfur are chemically bonded together / a chemical reaction has occurred [1]

The arrangement of atoms is different from the original elements, so the compound has a completely new set of properties / the chemical bonds create a new substance [1]

1 mark -- atoms are chemically bonded together or a chemical reaction has occurred 1 mark -- new arrangement of atoms gives new properties different from the elements

Question 2 -- Atomic Structure and Electronic Configurations

Total: 12 marks

A science teacher in London is preparing a lesson on atomic structure. She uses the periodic table to help students understand how atoms are built.

(a) [3]

Lithium has a proton number of 3 and a nucleon number of 7.

State the number of protons, neutrons and electrons in a lithium atom. Draw a labelled diagram showing the arrangement of these subatomic particles in a lithium atom.

Model Answer -- 2(a)

Protons = 3, Neutrons = 7 - 3 = 4, Electrons = 3 [1]

Nucleus in the centre containing 3 protons and 4 neutrons [1]

Electrons arranged in shells: 2 electrons in the first shell, 1 electron in the second shell (electronic configuration 2,1) [1]

1 mark -- correct number of protons 3 neutrons 4 electrons 3 1 mark -- nucleus containing protons and neutrons in centre 1 mark -- electrons in shells with correct configuration 2 then 1

(b) [4]

Complete the following table for the atoms shown.

Atom	Proton number	Nucleon number	Electrons	Electronic configuration
Sodium (Na)	11	23	?	?
Phosphorus (P)	?	31	15	?
Argon (Ar)	18	?	18	?

Note: Argon has nucleon number 40.

Model Answer -- 2(b)

Sodium: electrons = 11, electronic configuration = 2,8,1 [1]

Phosphorus: proton number = 15, electronic configuration = 2,8,5 [1]

Argon: nucleon number = 40, electronic configuration = 2,8,8 [1]

All four missing values correct with electronic configurations using correct shell filling rules (max 2 in first shell, max 8 in second and third shells) [1]

1 mark -- sodium has 11 electrons and configuration 2 8 1 1 mark -- phosphorus has proton number 15 and configuration 2 8 5 1 mark -- argon has nucleon number 40 and configuration 2 8 8 1 mark -- all electronic configurations follow correct shell filling rules

An element X has the electronic configuration 2,8,7.

(i) Identify element X.
(ii) State the group and period of element X in the periodic table.
(iii) Explain how the electronic configuration tells you the group and period.

Model Answer -- 2(c)

(i) Element X is chlorine (Cl), proton number 17 [1]

(ii) Group VII (or Group 17), Period 3 [1]

(iii) The number of electron shells = the period number (3 shells = Period 3). The number of electrons in the outer shell = the group number (7 outer electrons = Group VII) [1]

1 mark -- element X is chlorine 1 mark -- Group VII or 17 and Period 3 1 mark -- number of shells gives period and outer electrons give group

(d) [2]

The noble gases (Group 0/VIII) include helium (2), neon (2,8) and argon (2,8,8).

Explain, with reference to electronic configuration, why the noble gases are very unreactive.

Model Answer -- 2(d)

Noble gases have a full/complete outer shell of electrons (stable electronic configuration / stable octet, except helium which has a stable duplet) [1]

They have no tendency to gain, lose or share electrons, so they do not form bonds / do not react with other elements [1]

1 mark -- noble gases have full outer electron shells or stable octet 1 mark -- no tendency to gain lose or share electrons so do not react

Question 3 -- Isotopes

Total: 12 marks

A hospital in Singapore uses different isotopes in medical imaging and treatment. A chemistry student is learning about the concept of isotopes and their applications.

(a) [2]

Define the term isotopes.

Model Answer -- 3(a)

Isotopes are atoms of the same element / with the same proton number (Z) [1]

but with different numbers of neutrons / different nucleon numbers (A) [1]

1 mark -- same element or same proton number 1 mark -- different number of neutrons or different nucleon number

(b) [2]

Silicon has three naturally occurring isotopes: silicon-28, silicon-29 and silicon-30. The proton number of silicon is 14.

Write the nuclide notation for:
(i) silicon-28
(ii) silicon-29

Model Answer -- 3(b)

(i) ²&sup8;⁄&sub1;&sub4;Si -- nucleon number 28 at top left, proton number 14 at bottom left [1]

(ii) ²&sup9;⁄&sub1;&sub4;Si -- nucleon number 29 at top left, proton number 14 at bottom left [1]

1 mark -- correct nuclide notation for silicon-28 with 28 top and 14 bottom 1 mark -- correct nuclide notation for silicon-29 with 29 top and 14 bottom

The percentage abundances of the three silicon isotopes are:

Isotope	Mass number	Percentage abundance
Silicon-28	28	92.2%
Silicon-29	29	4.7%
Silicon-30	30	3.1%

Calculate the relative atomic mass (A_r) of silicon. Show your working clearly and give your answer to one decimal place.

Model Answer -- 3(c)

A_r = sum of (isotope mass x percentage abundance) / 100 [1]

A_r = (28 x 92.2) + (29 x 4.7) + (30 x 3.1) / 100 [1]

A_r = (2581.6 + 136.3 + 93.0) / 100 = 2810.9 / 100 [1]

A_r = 28.1 (to 1 decimal place) [1]

1 mark -- correct formula for relative atomic mass using abundances 1 mark -- correct substitution of values into formula 1 mark -- correct arithmetic for numerator 2810.9 1 mark -- final answer 28.1 to one decimal place

(d) [2]

Explain why all three isotopes of silicon have the same chemical properties.

Model Answer -- 3(d)

All three isotopes have the same number of electrons / same electronic configuration (2,8,4) [1]

Chemical properties depend on the electronic configuration / how electrons are arranged, not on the number of neutrons in the nucleus [1]

1 mark -- same number of electrons or same electronic configuration 1 mark -- chemical properties depend on electron arrangement not neutrons

(e) [2]

Although isotopes have the same chemical properties, they can have slightly different physical properties such as density and rate of diffusion.

Suggest why isotopes may differ in their rate of diffusion.

Model Answer -- 3(e)

Different isotopes have different masses / different numbers of neutrons make them heavier or lighter [1]

Heavier isotopes move more slowly at the same temperature / have lower average speed, so they diffuse more slowly [1]

1 mark -- different isotopes have different masses 1 mark -- heavier isotopes move more slowly so diffuse more slowly

Question 4 -- Ionic Bonding

Total: 12 marks

A materials scientist at a research institute in Cambridge is studying ionic compounds. She investigates how atoms of metals and non-metals combine to form ionic bonds.

(a) [3]

Magnesium (Mg) has the electronic configuration 2,8,2. Oxygen (O) has the electronic configuration 2,6.

Describe, in terms of electron transfer, how magnesium and oxygen atoms form ions when they react to form magnesium oxide (MgO). Include the charges on the ions formed.

Model Answer -- 4(a)

Magnesium atom loses its 2 outer shell electrons to form a Mg²+ ion (with electronic configuration 2,8) [1]

Oxygen atom gains these 2 electrons to form an O²− ion (with electronic configuration 2,8) [1]

Both ions now have a stable noble gas electronic configuration (same as neon) / full outer shell of electrons [1]

1 mark -- magnesium loses 2 outer electrons to form Mg2+ ion 1 mark -- oxygen gains 2 electrons to form O2- ion 1 mark -- both ions achieve stable noble gas configuration or full outer shell

(b) [3]

Draw a dot-and-cross diagram to show the ionic bonding in magnesium oxide (MgO). Show the outer electron shells only, the charges on the ions, and use dots for one element and crosses for the other.

Model Answer -- 4(b)

Mg²+ ion shown with no outer electrons (empty outer shell / all electrons transferred) with 2+ charge [1]

O²− ion shown with 8 electrons in outer shell (6 shown as crosses from oxygen, 2 shown as dots from magnesium) with 2- charge [1]

Square brackets around each ion with charges outside the brackets [1]

1 mark -- Mg2+ ion shown with empty outer shell and correct 2+ charge 1 mark -- O2- ion with 8 outer electrons using dots and crosses and correct 2- charge 1 mark -- square brackets around ions with charges shown outside

Calcium has the electronic configuration 2,8,8,2. Chlorine has the electronic configuration 2,8,7.

Draw a dot-and-cross diagram to show the ionic bonding in calcium chloride (CaCl₂). Show outer shells only.

Model Answer -- 4(c)

Ca²+ ion shown with empty outer shell and 2+ charge, in square brackets [1]

Two Cl− ions each with 8 electrons in outer shell (7 from Cl shown as crosses, 1 from Ca shown as a dot), each with 1− charge [1]

Correct ratio of one Ca²+ to two Cl− ions, showing that calcium transfers one electron to each chlorine atom [1]

1 mark -- Ca2+ ion with empty outer shell and correct 2+ charge in brackets 1 mark -- two Cl- ions each with 8 outer electrons using dots and crosses with 1- charge 1 mark -- correct ratio one calcium ion to two chloride ions showing electron transfer

(d) [3]

Magnesium oxide has a very high melting point (2852 °C). It does not conduct electricity when solid but does conduct when molten.

Explain these properties in terms of the structure and bonding in magnesium oxide.

Model Answer -- 4(d)

MgO has a giant ionic lattice structure with strong electrostatic forces of attraction between the oppositely charged Mg²+ and O²− ions in all directions. A large amount of energy is needed to overcome these strong forces, so the melting point is very high [1]

In the solid state, the ions are held in fixed positions in the lattice and cannot move, so they cannot carry charge / conduct electricity [1]

When molten, the ions are free to move throughout the liquid and can carry charge / act as mobile charge carriers, so the molten compound conducts electricity [1]

1 mark -- giant ionic lattice with strong electrostatic forces needing much energy to overcome 1 mark -- solid ions fixed in position cannot move so cannot conduct 1 mark -- when molten ions are free to move and carry charge so conducts

Question 5 -- Covalent Bonding

Total: 10 marks

Covalent compounds are found everywhere in daily life -- from the carbon dioxide we exhale to the ethene used to make plastic bags. A student in Delhi is studying how atoms share electrons to form covalent bonds.

(a) [1]

Define the term covalent bond.

Model Answer -- 5(a)

A covalent bond is a shared pair of electrons between two atoms [1]

1 mark -- shared pair of electrons between two atoms

(b) [3]

Carbon has 4 outer electrons. Oxygen has 6 outer electrons.

Draw a dot-and-cross diagram for a molecule of carbon dioxide (CO₂). Show the outer electron shells only. Carbon dioxide contains two carbon-oxygen double bonds.

Model Answer -- 5(b)

Each oxygen atom shares two pairs of electrons with the central carbon atom (double bond = two shared pairs) [1]

Carbon has 8 electrons around it (4 of its own + 4 shared from the two oxygens), each oxygen has 8 electrons around it (4 lone pair + 2 own bonding + 2 shared from carbon) [1]

Two lone pairs on each oxygen correctly shown, with dots and crosses used to distinguish atoms [1]

1 mark -- two double bonds shown between carbon and each oxygen with correct shared pairs 1 mark -- all atoms have 8 electrons in outer shell completing the octet 1 mark -- two lone pairs on each oxygen and dots and crosses used correctly

Ethene (C₂H₄) contains a carbon-carbon double bond. Each carbon atom is also bonded to two hydrogen atoms.

Draw a dot-and-cross diagram for a molecule of ethene. Show the outer electron shells only.

Model Answer -- 5(c)

Carbon-carbon double bond shown correctly (two shared pairs of electrons between the two carbon atoms) [1]

Each carbon bonded to two hydrogen atoms with single covalent bonds (one shared pair each), giving each carbon 8 electrons in its outer shell [1]

Dots and crosses used correctly to distinguish electrons from different atoms; each hydrogen has 2 electrons around it (shared pair) [1]

1 mark -- carbon-carbon double bond with two shared electron pairs 1 mark -- each carbon bonded to two hydrogens with single bonds giving octet on each carbon 1 mark -- dots and crosses used correctly with each hydrogen having 2 electrons

(d) [3]

Carbon dioxide (CO₂) is a gas at room temperature with a very low boiling point (-78 °C). However, silicon dioxide (SiO₂), which also contains covalent bonds, has a very high melting point (1710 °C).

Explain why CO₂ has a very low boiling point while SiO₂ has a very high melting point, even though both contain covalent bonds.

Model Answer -- 5(d)

CO₂ exists as simple/small individual molecules with weak intermolecular forces (forces between molecules). Only a small amount of energy is needed to overcome these weak forces, so the boiling point is very low [1]

SiO₂ has a giant covalent structure where every silicon atom is bonded to four oxygen atoms in a continuous 3D network / there are no individual molecules [1]

In SiO₂, many strong covalent bonds must be broken to melt it, which requires a very large amount of energy, giving it a very high melting point [1]

1 mark -- CO2 has simple molecules with weak intermolecular forces needing little energy 1 mark -- SiO2 has giant covalent structure with continuous 3D network 1 mark -- many strong covalent bonds must be broken in SiO2 needing much energy

Question 6 -- Giant Covalent Structures

Total: 12 marks

A materials engineer in Bengaluru is selecting materials for different industrial applications. She is comparing the structures and properties of diamond, graphite, and silicon dioxide.

(a) [4]

Diamond is an allotrope of carbon. It is the hardest naturally occurring substance known.

(i) Describe the structure of diamond.
(ii) Use the structure of diamond to explain why it is very hard and has a very high melting point.

Model Answer -- 6(a)

(i) Diamond has a giant covalent structure / giant molecular structure / macromolecular structure [1]

Each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a rigid 3D network [1]

(ii) Diamond is very hard because the rigid 3D network of strong covalent bonds extends in all directions, so there are no layers that can slide [1]

It has a very high melting point because many strong covalent bonds must be broken to melt it, which requires a very large amount of energy [1]

1 mark -- giant covalent or macromolecular structure 1 mark -- each carbon bonded to four others in tetrahedral arrangement or 3D network 1 mark -- very hard because rigid 3D network of strong bonds in all directions no layers 1 mark -- high melting point because many strong covalent bonds need large energy to break

(b) [4]

Graphite is another allotrope of carbon. It is used as a lubricant and in pencil leads. Unlike diamond, graphite is soft and slippery, and it conducts electricity.

(i) Describe the structure of graphite.
(ii) Explain why graphite is soft and slippery.
(iii) Explain why graphite can conduct electricity.

Model Answer -- 6(b)

(i) Graphite has a giant covalent layered structure. Each carbon atom is bonded to three other carbon atoms in flat hexagonal layers [1]

The layers are held together by weak intermolecular forces (weak van der Waals forces between the layers) [1]

(ii) Graphite is soft and slippery because the weak forces between layers allow the layers to slide over each other easily [1]

(iii) Each carbon atom uses only 3 of its 4 outer electrons for bonding, so each carbon has one delocalised / free electron. These delocalised electrons can move along the layers and carry charge, allowing graphite to conduct electricity [1]

1 mark -- each carbon bonded to three others in flat hexagonal layers 1 mark -- weak intermolecular forces or weak van der Waals forces between layers 1 mark -- soft and slippery because weak forces allow layers to slide over each other 1 mark -- delocalised electrons from fourth outer electron can move along layers and carry charge

Silicon dioxide (SiO₂) is used in optical fibres and in the manufacture of glass. It has a melting point of 1710 °C and does not conduct electricity.

(i) Describe the structure of silicon dioxide.
(ii) Explain why silicon dioxide has a very high melting point.
(iii) Explain why silicon dioxide does not conduct electricity.

Model Answer -- 6(c)

(i) Silicon dioxide has a giant covalent structure similar to diamond. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms, forming a continuous 3D network [1]

(ii) It has a very high melting point because there are many strong covalent bonds throughout the giant structure, and a very large amount of energy is needed to break these bonds [1]

(iii) All electrons are involved in covalent bonding / there are no free or delocalised electrons to carry charge [1]

There are no ions present, so there are no mobile charge carriers at all [1]

1 mark -- giant covalent structure with each silicon bonded to four oxygens in 3D network 1 mark -- high melting point because many strong covalent bonds need much energy to break 1 mark -- no free or delocalised electrons available to carry charge 1 mark -- no ions present so no mobile charge carriers

Question 7 -- Metallic Bonding

Total: 10 marks

India is one of the world's largest producers of steel. An engineer at the Tata Steel plant in Jamshedpur is explaining the properties of metals to a group of visiting students.

(a) [3]

Describe metallic bonding. Include in your answer a description of the structure and the role of electrons.

Model Answer -- 7(a)

In a metal, the atoms lose their outer shell electrons to form a lattice / regular arrangement of positive metal ions (cations) [1]

The outer electrons become delocalised / free to move throughout the whole structure (they are no longer associated with any particular metal atom) [1]

Metallic bonding is the strong electrostatic attraction between the positive metal ions and the sea / cloud of delocalised electrons [1]

1 mark -- regular lattice of positive metal ions or cations 1 mark -- outer electrons become delocalised or free to move throughout structure 1 mark -- strong electrostatic attraction between positive ions and delocalised electrons

(b) [2]

Copper is widely used for electrical wiring.

Use your knowledge of metallic bonding to explain why metals such as copper are good conductors of electricity.

Model Answer -- 7(b)

Metals contain delocalised / free electrons that are not attached to any particular atom [1]

When a voltage / potential difference is applied, these delocalised electrons can move through the metal structure towards the positive terminal, carrying the electric charge / current [1]

1 mark -- metals contain delocalised or free electrons 1 mark -- electrons move through the structure carrying charge or current when voltage applied

Metals are malleable, which means they can be hammered into different shapes without breaking. This is why steel can be pressed into car body panels at the factory.

Use your knowledge of metallic bonding to explain why metals are malleable.

Model Answer -- 7(c)

When a force is applied, the layers of positive metal ions can slide over each other into new positions [1]

The delocalised electrons can adjust / re-form the metallic bonds in the new positions, so the structure does not break / the bonding is maintained [1]

1 mark -- layers of metal ions can slide over each other when force applied 1 mark -- delocalised electrons re-form bonds in new positions so metal does not break

(d) [3]

Both metals and ionic compounds contain positive ions arranged in a lattice. However, metals can conduct electricity in the solid state, while ionic compounds cannot.

State three differences between metallic bonding and ionic bonding.

Model Answer -- 7(d)

In metallic bonding, electrons are delocalised / shared across all atoms; in ionic bonding, electrons are transferred from one atom to another [1]

Metallic bonding involves attraction between positive ions and delocalised electrons; ionic bonding involves attraction between positive cations and negative anions [1]

Metals conduct electricity in the solid state (due to mobile delocalised electrons); ionic compounds only conduct when molten or dissolved (when ions become mobile) [1]

1 mark -- metallic has delocalised electrons while ionic has electron transfer 1 mark -- metallic bonding involves positive ions and electron sea while ionic involves cations and anions 1 mark -- metals conduct as solid but ionic only conducts when molten or dissolved

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Total Marks

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A* : 56+

A : 48-55

B : 40-47

C : 32-39

Below C : <32

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