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IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 2: Atoms, Elements and Compounds -- Mock Exam 1
1 hour 15 minutes
80
7
75:00
0620

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Question 1 -- Elements, Compounds and Mixtures / Atomic Structure
Total: 12 marks
A chemistry teacher in Leeds shows her class five substances: iron filings, sodium chloride, air, distilled water and sulfur powder.
(a) [3]
Classify each of the following substances as an element, a compound or a mixture.

(i) Iron filings
(ii) Sodium chloride
(iii) Air
Model Answer -- 1(a)
Iron filings: element [1]
Sodium chloride: compound [1]
Air: mixture [1]
(b) [3]
(i) State the difference between a compound and a mixture. [2]

(ii) State one way in which an element is different from a compound. [1]
Model Answer -- 1(b)
A compound is formed by a chemical reaction / has a fixed composition, whereas a mixture is not chemically combined / has no fixed composition [1]
The components of a compound cannot be separated by physical methods, whereas the components of a mixture can be separated by physical methods [1]
An element contains only one type of atom, whereas a compound contains two or more different types of atoms chemically combined [1]
(c) [3]
Complete the table to show the relative charge and relative mass of each sub-atomic particle.

Particle Relative charge Relative mass
Proton ....... 1
Neutron 0 .......
Electron ....... .......
Model Answer -- 1(c)
Proton relative charge: +1 [1]
Neutron relative mass: 1 [1]
Electron relative charge: −1 AND relative mass: 1/1836 (or negligible / very small) [1]
(d) [3]
A sodium atom has a proton number (atomic number) of 11 and a nucleon number (mass number) of 23.

(i) State the number of protons, neutrons and electrons in a sodium atom. [2]

(ii) Define the term proton number. [1]
Model Answer -- 1(d)
Protons = 11, Electrons = 11 [1]
Neutrons = 23 − 11 = 12 [1]
Proton number is the number of protons in the nucleus of an atom [1]
Question 2 -- Atomic Structure and Electronic Configurations
Total: 12 marks
A student in Bangalore studies the first 20 elements of the Periodic Table and their electronic configurations.
(a) [4]
Write the electronic configuration of each of the following atoms.

(i) Carbon (Z = 6)
(ii) Aluminium (Z = 13)
(iii) Argon (Z = 18)
(iv) Calcium (Z = 20)
Model Answer -- 2(a)
Carbon: 2, 4 [1]
Aluminium: 2, 8, 3 [1]
Argon: 2, 8, 8 [1]
Calcium: 2, 8, 8, 2 [1]
(b) [3]
Using your answers to part (a):

(i) Explain how the electronic configuration of an element relates to its group number in the Periodic Table. [1]

(ii) Explain how the electronic configuration of an element relates to its period number in the Periodic Table. [1]

(iii) State the group and period of aluminium. [1]
Model Answer -- 2(b)
The group number equals the number of electrons in the outer shell / outermost energy level [1]
The period number equals the number of occupied electron shells / energy levels [1]
Aluminium is in Group III / Group 13 and Period 3 [1]
(c) [2]
Draw the electronic structure (electron shell diagram) of a silicon atom (Z = 14). Show the nucleus and the arrangement of electrons in shells.
Nucleus Draw the correct number of electrons on each shell
Model Answer -- 2(c)
14p 14n e e e e e e e e e e e e e e
Silicon: 2, 8, 4 -- correct number of electrons in each shell [1]
Nucleus labelled with 14p and 14n (or just labelled correctly) [1]
(d) [3]
An element X has the electronic configuration 2, 8, 7.

(i) Identify element X. [1]

(ii) State the group and period of element X. [1]

(iii) Predict whether element X is a metal or a non-metal. Explain your answer. [1]
Model Answer -- 2(d)
Element X is chlorine (Cl) -- proton number = 2 + 8 + 7 = 17 [1]
Group VII / Group 17, Period 3 [1]
Non-metal, because it is in Group VII / has 7 outer electrons so it gains electrons rather than losing them / is on the right side of the Periodic Table [1]
Question 3 -- Isotopes
Total: 12 marks
Copper is widely used in electrical wiring throughout India and the United Kingdom. Natural copper consists of two isotopes: 63Cu and 65Cu.
(a) [2]
Define the term isotopes.
Model Answer -- 3(a)
Isotopes are atoms of the same element [1]
with the same number of protons but different numbers of neutrons / different mass numbers [1]
(b) [3]
Copper has a proton number of 29.

(i) State the number of protons, neutrons and electrons in an atom of 63Cu. [2]

(ii) Write the full nuclide notation for the isotope copper-65. [1]
Model Answer -- 3(b)
Protons = 29, Electrons = 29 [1]
Neutrons = 63 − 29 = 34 [1]
6529Cu -- mass number 65 at top, proton number 29 at bottom [1]
(c) [2]
Explain why the two isotopes of copper have the same chemical properties.
Model Answer -- 3(c)
Both isotopes have the same number of electrons / the same electronic configuration [1]
Chemical properties depend on the arrangement of electrons / the number of outer shell electrons, which is the same in both isotopes [1]
(d) [3]
A sample of copper contains 69.2% of 63Cu and 30.8% of 65Cu.

Calculate the relative atomic mass (Ar) of copper. Give your answer to one decimal place.
Model Answer -- 3(d)
Ar = (69.2 × 63) + (30.8 × 65) / 100 [1] -- correct formula set up
Ar = (4359.6 + 2002.0) / 100 = 6361.6 / 100 [1] -- correct working
Ar = 63.6 [1] -- correct final answer to 1 decimal place
(e) [2]
Suggest one physical property in which the two isotopes of copper would differ. Explain your answer.
Model Answer -- 3(e)
Density / rate of diffusion / mass [1] -- any valid physical property
65Cu is heavier / has more neutrons / greater mass, so it would have a higher density / diffuse more slowly [1]
Question 4 -- Ionic Bonding
Total: 12 marks
Magnesium oxide (MgO) is used in fire bricks in steel furnaces across Sheffield. It has a very high melting point of 2852 °C.
(a) [3]
(i) Define the terms cation and anion. [2]

(ii) State the charge on a magnesium ion and an oxide ion. [1]
Model Answer -- 4(a)
A cation is a positively charged ion / an ion formed by loss of electrons [1]
An anion is a negatively charged ion / an ion formed by gain of electrons [1]
Magnesium ion: Mg2+, Oxide ion: O2− [1]
(b) [3]
(i) Define an ionic bond. [1]

(ii) Describe what happens to the electrons when magnesium reacts with oxygen to form magnesium oxide. [2]
Model Answer -- 4(b)
An ionic bond is the electrostatic attraction between oppositely charged ions / a strong force of attraction between a positive ion and a negative ion [1]
Magnesium atom loses 2 electrons from its outer shell to form Mg2+ [1]
Oxygen atom gains these 2 electrons into its outer shell to form O2− / to achieve a stable octet [1]
(c) [3]
Draw a dot-and-cross diagram to show the ionic bonding in magnesium oxide (MgO). Show the charges on the ions and the electronic configuration of the ions formed.

Mg: 2, 8, 2    O: 2, 6
Draw the dot-and-cross diagram for MgO showing electron transfer Mg ion O ion
Model Answer -- 4(c)
Mg 2+ x x x x x x x x O 2−
Mg2+ shown with electronic configuration 2, 8 (lost 2 outer electrons) [1]
O2− shown with electronic configuration 2, 8 (gained 2 electrons) [1]
Transferred electrons clearly distinguished (dots vs crosses) with correct charges on ions [1]
(d) [3]
Magnesium oxide has a high melting point and conducts electricity when molten but not when solid.

(i) Describe the structure of magnesium oxide. [1]

(ii) Explain why magnesium oxide has a high melting point. [1]

(iii) Explain why magnesium oxide conducts electricity when molten but not when solid. [1]
Model Answer -- 4(d)
Magnesium oxide has a giant ionic lattice structure -- a regular arrangement of alternating Mg2+ and O2− ions [1]
High melting point because there are strong electrostatic forces of attraction between the oppositely charged ions, which require a large amount of energy to overcome [1]
When solid, ions are held in fixed positions and cannot move to carry charge; when molten, the ions are free to move and carry the electric current [1]
Question 5 -- Covalent Bonding
Total: 10 marks
Water is essential for life. A researcher at the Indian Institute of Science in Bengaluru studies the bonding in water and other simple covalent molecules.
(a) [1]
Define a covalent bond.
Model Answer -- 5(a)
A covalent bond is a shared pair of electrons between two atoms [1]
(b) [4]
Draw dot-and-cross diagrams to show the covalent bonding in each of the following molecules. Show outer shell electrons only.

(i) Water, H2O [1]
(ii) Methane, CH4 [1]
(iii) Carbon dioxide, CO2 [1]
(iv) Nitrogen, N2 [1]
Model Answer -- 5(b)

(i) Water, H2O

O H H x x 2 bonding pairs + 2 lone pairs on O

(ii) Methane, CH4

C H x H x H x H x 4 bonding pairs, no lone pairs on C

(iii) Carbon dioxide, CO2 (double bonds)

C O O x x x x Two double bonds (O=C=O), 2 lone pairs on each O

(iv) Nitrogen, N2 (triple bond)

N N x x x Triple bond (3 shared pairs), 1 lone pair on each N
H2O: 2 shared pairs between O and H atoms, 2 lone pairs on O, each H has 2 electrons [1]
CH4: 4 shared pairs between C and H atoms, C achieves an octet [1]
CO2: 2 double bonds (O=C=O), each O has 2 lone pairs, C achieves an octet [1]
N2: triple bond (3 shared pairs), each N has 1 lone pair [1]
(c) [3]
Simple covalent substances such as water and methane have low melting and boiling points.

(i) State whether simple covalent molecules conduct electricity. [1]

(ii) Explain why simple covalent substances have low boiling points. [2]
Model Answer -- 5(c)
Simple covalent molecules do not conduct electricity (as solids, liquids or in solution) because they have no free ions or delocalised electrons to carry the current [1]
The intermolecular forces (forces between molecules) are weak [1]
Only a small amount of energy is needed to overcome these weak intermolecular forces, so the boiling point is low (note: the strong covalent bonds within molecules are NOT broken) [1]
(d) [2]
Methanol has the molecular formula CH3OH.

(i) State the total number of covalent bonds in one molecule of methanol. [1]

(ii) State the total number of lone pairs of electrons in one molecule of methanol. [1]
Model Answer -- 5(d)
5 covalent bonds (3 C–H bonds + 1 C–O bond + 1 O–H bond) [1]
2 lone pairs (both on the oxygen atom) [1]
Question 6 -- Giant Covalent Structures
Total: 12 marks
Diamond and graphite are both allotropes of carbon. They are used in drill bits for mining in South Africa and as lubricants in heavy machinery, respectively.
(a) [3]
(i) Describe the structure of diamond. [2]

(ii) State one use of diamond, linked to its properties. [1]
Diamond structure C C C C C Each C bonded to 4 others in a tetrahedral arrangement
Model Answer -- 6(a)
Diamond has a giant covalent structure / giant molecular structure / macromolecular structure [1]
Each carbon atom is bonded to four other carbon atoms by strong covalent bonds in a tetrahedral arrangement [1]
Use: cutting tools / drill bits / jewellery -- because diamond is the hardest natural substance / very hard [1]
(b) [4]
(i) Describe the structure of graphite. [3]

(ii) State one use of graphite, linked to its properties. [1]
Graphite structure weak forces Layers of hexagonal rings, each C bonded to 3 others Weak forces between layers -- layers can slide
Model Answer -- 6(b)
Graphite has a giant covalent / layered structure with each carbon atom bonded to three other carbon atoms in flat hexagonal layers [1]
There are weak intermolecular forces / weak van der Waals forces between the layers, so the layers can slide over each other [1]
Each carbon atom has one delocalised electron which is free to move between the layers / along the layers, so graphite conducts electricity [1]
Use: lubricant (because layers slide) / electrodes (because it conducts electricity) / pencil leads (because layers slide off onto paper) [1]
(c) [2]
(i) Explain why diamond does not conduct electricity but graphite does. [1]

(ii) Explain why both diamond and graphite have very high melting points. [1]
Model Answer -- 6(c)
In diamond all four outer electrons of each carbon are used in covalent bonds so there are no delocalised / free electrons; in graphite each carbon uses only three electrons for bonding, leaving one electron delocalised and free to move and carry current [1]
Both have many strong covalent bonds throughout the structure which require a large amount of energy to break [1]
(d) [3]
Silicon dioxide (SiO2) is found in sand and is used to make glass.

(i) Describe the structure of silicon dioxide. [2]

(ii) Predict the melting point of silicon dioxide (high or low) and explain your answer. [1]
Model Answer -- 6(d)
Silicon dioxide has a giant covalent structure similar to diamond [1]
Each silicon atom is bonded to four oxygen atoms by strong covalent bonds, and each oxygen atom is bonded to two silicon atoms, forming a three-dimensional network [1]
High melting point because there are many strong covalent bonds throughout the structure that require a large amount of energy to break [1]
Question 7 -- Metallic Bonding
Total: 10 marks
Copper is widely used for electrical wiring in homes across Mumbai. Aluminium is used in aircraft manufacturing by companies such as Airbus in Toulouse, France. Both are metals with characteristic properties.
(a) [3]
Describe metallic bonding. You may include a labelled diagram to support your answer.
Metallic bonding model M+ M+ M+ M+ M+ M+ M+ M+ M+ M+ M+ e e e e e e e e e e e e e e
Model Answer -- 7(a)
Metal atoms lose their outer shell electrons to form positive ions / cations [1]
These electrons become delocalised / free to move throughout the structure, forming a "sea" of electrons [1]
There is a strong electrostatic attraction between the positive metal ions and the delocalised electrons -- this is the metallic bond [1]
(b) [2]
Explain why metals are good conductors of electricity.
Model Answer -- 7(b)
Metals contain delocalised electrons that are free to move throughout the metal structure [1]
When a potential difference / voltage is applied, these electrons drift towards the positive terminal, carrying the electric current [1]
(c) [3]
Metals are described as malleable (can be hammered into shape) and ductile (can be drawn into wires).

(i) Define the term malleable. [1]

(ii) Explain, in terms of structure and bonding, why metals are malleable. [2]
Model Answer -- 7(c)
Malleable means the metal can be hammered or pressed into different shapes without breaking [1]
The layers of positive ions / metal ions can slide over each other when a force is applied [1]
The delocalised electrons continue to hold the ions together / the metallic bond is maintained in the new positions, so the metal does not shatter [1]
(d) [2]
The table below compares three types of bonding. Complete the table.

Property Ionic Simple covalent Metallic
Conducts electricity as solid? No No .......
Melting point ....... Low High
Model Answer -- 7(d)
Metallic solid conducts electricity: Yes [1]
Ionic melting point: High [1]

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IGCSE Chemistry 0620 -- Paper 4 Theory (Extended) -- Topic 2: Atoms, Elements and Compounds -- Mock Exam 1
For examination preparation only. Not an official Cambridge Assessment paper.