Atomic Structure Exam Questions
Practice IGCSE Chemistry exam questions on atomic structure. Covers protons, neutrons, electrons, electron configuration, atomic number, mass number, and sub-atomic particles with mark schemes and examiner notes.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
These questions cover all aspects of atomic structure tested in the 0620 exam. Write each answer fully before checking.
Question 1 (3 marks, Core)
Complete the table below for the three sub-atomic particles.
| Particle | Relative mass | Relative charge | Location in atom |
|---|---|---|---|
| Proton | ? | ? | ? |
| Neutron | ? | ? | ? |
| Electron | ? | ? | ? |
Mark scheme
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus [1] |
| Neutron | 1 | 0 | Nucleus [1] |
| Electron | 1/1840 (or negligible) | -1 | Shells/orbits around the nucleus [1] |
Examiner note: The electron mass is often stated as “very small” or “negligible” or 1/1840. Writing “0” for electron mass is not accepted — electrons do have mass, just very little compared to protons and neutrons.
Question 2 (3 marks, Core)
An atom of aluminium has the symbol $^{27}_{13}$Al.
(a) State the atomic number and mass number of this atom. (1)
(b) Calculate the number of protons, neutrons, and electrons in this atom. (2)
Mark scheme
(a) Atomic number = 13, mass number = 27 [1]
(b) Protons = 13 [1] (same as atomic number) Neutrons = 27 - 13 = 14 [1] Electrons = 13 (same as protons in a neutral atom)
Examiner note: “Number of electrons = number of protons” only applies to neutral atoms, not ions. If the question specifies an ion, adjust accordingly. Here the question says “atom,” so it is neutral.
Question 3 (2 marks, Core)
Draw the electron configuration (electron shell diagram) of a sodium atom (atomic number 11).
Mark scheme
- 2 electrons in the first shell, 8 electrons in the second shell, 1 electron in the third shell [1]
- Correctly drawn with nucleus labelled and electrons shown in correct shells (2, 8, 1) [1]
Examiner note: A common error is placing 8 electrons in the first shell. The maximum electrons per shell are: first = 2, second = 8, third = 8 (for IGCSE purposes). Write the configuration as 2,8,1.
Question 4 (3 marks, Core)
The electronic configurations of four atoms are shown below:
- P: 2, 1
- Q: 2, 8, 7
- R: 2, 8, 8
- S: 2, 8, 2
(a) Which atom is in Group I of the periodic table? (1)
(b) Which atom is a noble gas? (1)
(c) Which atom would form an ion with a charge of 2+? (1)
Mark scheme
(a) P [1] — it has 1 electron in its outer shell
(b) R [1] — it has a full outer shell (8 electrons)
(c) S [1] — it has 2 electrons in its outer shell, which it would lose to form a 2+ ion
Examiner note: The group number equals the number of outer shell electrons (for main group elements). A full outer shell (2 for helium, 8 for others) indicates a noble gas. These are fundamental links between electron configuration and the periodic table.
Question 5 (4 marks, Supplement)
An atom of element X has 19 protons, 20 neutrons and 19 electrons.
(a) Write the electronic configuration of element X. (1)
(b) State the group and period of element X in the periodic table. (2)
(c) Predict the charge on the ion that element X would form. Explain your answer. (1)
Mark scheme
(a) 2, 8, 8, 1 [1]
(b) Group I [1], Period 4 [1]
(c) 1+ ion [1] — it loses the one outer electron to achieve a stable electron configuration (full outer shell of 8)
Examiner note: The period number equals the number of occupied electron shells. Element X is potassium (K). Candidates who get the electron configuration wrong in (a) will get (b) and (c) wrong too — this is an error-carried-forward situation, so get (a) right first.
Question 6 (2 marks, Core)
Explain why atoms are electrically neutral.
Mark scheme
- Atoms contain equal numbers of protons and electrons [1]
- The positive charges of the protons are balanced/cancelled by the negative charges of the electrons [1]
Examiner note: “Atoms have no charge” restates the question. You must explain why — equal numbers of protons (+) and electrons (-).
Question 7 (3 marks, Supplement)
The table shows information about four particles, A to D.
| Particle | Protons | Neutrons | Electrons |
|---|---|---|---|
| A | 11 | 12 | 11 |
| B | 11 | 12 | 10 |
| C | 12 | 12 | 12 |
| D | 11 | 13 | 11 |
(a) Which two particles are isotopes of the same element? Explain your answer. (2)
(b) Which particle is a positive ion? Explain your answer. (1)
Mark scheme
(a) A and D [1] — they have the same number of protons (same atomic number = same element) but different numbers of neutrons (different mass numbers) [1]
(b) B [1] — it has more protons (11) than electrons (10), so it has an overall positive charge / it is a cation
Examiner note: Isotopes must have the same number of protons, not just the same number of electrons. A and C are different elements (different proton numbers) even though they have the same number of neutrons.
Question 8 (4 marks, Supplement)
An atom of phosphorus has the symbol $^{31}_{15}$P.
(a) State the number of protons, neutrons, and electrons in this atom. (2)
(b) Write the electronic configuration of phosphorus. (1)
(c) The phosphide ion has the formula P^{3-}. State how many electrons this ion has, and explain why. (1)
Mark scheme
(a) Protons = 15, electrons = 15 [1], neutrons = 31 - 15 = 16 [1]
(b) 2, 8, 5 [1]
(c) 18 electrons [1] — the atom gains 3 electrons to achieve a stable electron configuration with a full outer shell (2, 8, 8), giving it 3 more electrons than protons and hence a 3- charge.
Examiner note: A 3- ion means 3 extra electrons have been gained. Protons do not change when forming ions — only the number of electrons changes.
What to revise if you scored below 6
If sub-atomic particle questions caused problems, memorise the proton-neutron-electron table from atomic structure. If electron configurations were difficult, practise filling shells in the 2, 8, 8 pattern for the first 20 elements. Remember: atomic number = protons = electrons (for neutral atoms), and neutrons = mass number - atomic number.
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