OxfordAQA IGCSE Chemistry: Atomic Structure and the Periodic Table — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Atomic Structure and the Periodic Table study guide, Atomic Structure and the Periodic Table revision notes

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Section A

1. State the relative charge and relative mass of a proton, a neutron and an electron. [3]

2. Define isotope precisely. [2]

Section B

3. An atom of potassium is represented as ³⁹₁₉K. Calculate the number of protons, electrons and neutrons it contains. [3]

4. A second isotope of potassium has a mass number of 41. State how many neutrons this isotope has, and explain why it is still potassium rather than a different element. [3]

5. An atom has the electron structure 2,8,2. (a) State its total number of electrons. (b) State which group of the periodic table it belongs to, giving a reason. [3]

6. Explain, in terms of electron arrangement, why the noble gases (Group 0) are unreactive. [2]

Section C

7. A solid is heated until it melts, and then heated further until it boils. Explain, in terms of particle energy and arrangement, what happens at each change of state. [4]

8. Bromine vapour is released at one end of a gas jar and, over several minutes, spreads until it evenly colours the whole jar, even though the air inside was not stirred. Explain what this observation provides evidence for. [3]

9. Element X has an atomic number of 15. Determine its electron structure, showing your working, and state which group of the periodic table it belongs to. [4]

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Answers

1. Proton: relative charge +1, relative mass 1 [1]. Neutron: relative charge 0, relative mass 1 [1]. Electron: relative charge −1, relative mass very small [1].

2. An isotope is an atom of the same element (the same atomic number, so the same number of protons) with a different number of neutrons, and therefore a different mass number [1] [1].

3. Protons = 19 (equal to the atomic number) [1]. Electrons = 19 (equal to protons, since the atom is neutral) [1]. Neutrons = 39 − 19 = 20 (mass number minus atomic number) [1].

4. Neutrons = 41 − 19 = 22 [1]. It is still potassium because it has the same atomic number (19 protons) as the first isotope [1] — an element is defined by its number of protons, and only the number of neutrons has changed [1].

5. (a) 2 + 8 + 2 = 12 electrons [1]. (b) Group 2 [1], because the number of electrons in the outer shell (2) equals the group number for Groups 1–7 [1].

6. Noble gases already have a stable, full outer electron shell (eight outer electrons, except helium which has two) [1], so they have no tendency to gain, lose or share electrons with other atoms, which is why they do not readily react [1].

7. When the solid melts, particles gain enough energy to partly overcome the forces holding them in a fixed arrangement, so they can move past each other while remaining close together [1] [1]. When the liquid boils, particles gain enough energy to overcome the remaining forces between them almost entirely, so they spread apart to fill their container as a gas [1] [1].

8. This is evidence that the particles making up the bromine vapour are in constant, random motion [1], which allows them to spread out and mix with the particles of air without needing to be stirred or forced [1]; this process — particles spreading from an area of higher concentration to an area of lower concentration — is called diffusion [1].

9. Total electrons = atomic number = 15 [1]. Filling shells in order 2, 8, then the remainder: shell 1 = 2, shell 2 = 8, shell 3 = 15 − 2 − 8 = 5 [1], giving an electron structure of 2,8,5 [1]. Since the outer shell has 5 electrons, the element is in Group 5 [1].

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Where marks are usually lost

• Confusing atomic number (protons only) with mass number (protons plus neutrons) when calculating sub-atomic particle counts.
• Describing isotopes as having a different atomic number instead of a different number of neutrons.
• Forgetting that a neutral atom always has equal numbers of protons and electrons.
• Miscounting electron shells by not filling the lowest available energy level (2, then 8) before moving to the next shell.
• Stating that noble gases are unreactive without explaining this in terms of their stable, full outer electron shell.
• Describing a diffusion observation without linking it explicitly to constant, random particle motion.

Approaching Atomic Structure questions

Almost every calculation question on this topic reduces to the same three-step relationship — protons equal the atomic number, electrons equal the number of protons in a neutral atom, and neutrons equal the mass number minus the atomic number — so practising this until it is automatic pays off across the whole topic, not just questions that use the word “calculate.” For electron-structure questions, always fill the shells in order (2, then 8, then the remainder) rather than trying to recall an individual element’s structure from memory, since this method works directly for any of the first twenty elements and also gives the group number immediately, as the final digit of the electron structure, for Groups 1–7 (Group 0 is the exception: helium ends in 2 and neon and argon end in 8, since a full outer shell does not follow the same pattern). Diffusion and states-of-matter questions are marked on explanation, not description — a strong answer always connects what is observed back to particle energy, arrangement or motion, rather than simply naming the process.