Atomic Structure: Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Atomic Structure revision notes

Tier note: questions marked (0620 Extended, 5070 required) — calculating relative atomic mass from isotopic abundances — go beyond 0620 Core. All other questions are answerable by a 0620 Core candidate.

---

Questions

1. State the difference between an element, a compound and a mixture. [3]

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

3. Define the terms proton number and mass number. [2]

4. Define the term isotope, and explain why isotopes of the same element have identical chemical properties. [3]

5. A magnesium atom has proton number 12.

(a) Write its electronic configuration. [1] (b) State which group and which period this places magnesium in, explaining how you know. [2]

6. Write the electronic configuration of a chloride ion, Cl⁻ (proton number 17), and explain how it differs from the configuration of a chlorine atom. [2]

7. (0620 Extended, 5070 required) Copper exists as two isotopes, ⁶³Cu (69% abundance) and ⁶⁵Cu (31% abundance). Calculate the relative atomic mass of copper, giving your answer to one decimal place. [3]

8. A student is given a sample and told it is either a pure compound or a mixture of two substances.

(a) Suggest one physical method the student could use to help decide, and state what result would indicate a mixture. [2] (b) State one way in which a mixture’s composition differs from a compound’s. [1]

9. Write the isotope symbol notation for an atom of carbon-14 (mass number 14, proton number 6), and explain what each number represents. [2]

10. Determine the electronic configuration of a potassium atom (proton number 19), and state which group and period it belongs to. [3]

11. Sodium (proton number 11) has electronic configuration 2,8,1 as a neutral atom. Write the configuration of the sodium ion Na⁺, and explain the change. [2]

12. Explain why Group VIII noble gases are unreactive, in terms of electronic configuration. [2]

---

13. (0620 Extended, 5070 required) A sample of gallium contains 60.1% of the isotope gallium-69 and 39.9% of the isotope gallium-71. Calculate the relative atomic mass of gallium, to one decimal place. [2]

Answers

1. An element contains only one type of atom [1]. A compound is two or more elements chemically combined together in a fixed ratio [1]. A mixture contains two or more substances not chemically combined, which can be separated by physical means [1].

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

3. Proton number is the number of protons in the nucleus [1]. Mass number is the total number of protons and neutrons in the nucleus [1].

4. Isotopes are atoms of the same element (same proton number) with different numbers of neutrons [1] (and therefore different mass numbers) [1]. They have the same number of electrons, and therefore the same electronic configuration, and since chemical properties depend on electron arrangement, their chemical properties are identical [1].

5. (a) 2,8,2 [1]. (b) Group II, because the outer shell has 2 electrons [1]; Period 3, because 3 shells are occupied [1].

6. Chlorine atom: 2,8,7. Chloride ion: 2,8,8 [1]. The ion has gained one electron to fill its outer shell, while the number of protons (and therefore the element) stays the same [1].

7. (0620 Extended, 5070 required) Ar = (63 × 69 + 65 × 31) / 100 [1] = (4347 + 2015) / 100 = 6362 / 100 [1] = 63.6 [1].

8. (a) Compare the sample’s melting or boiling point with the known value for the pure compound [1]; a mixture melts/boils over a range of temperatures rather than at a single sharp point [1]. (b) A mixture’s composition can vary (not a fixed ratio), whereas a compound always has the same fixed ratio of its constituent elements [1]. Any other valid physical method (e.g. chromatography for a mixture of soluble substances) should be credited in (a).

9. ¹⁴₆C [1]. The top number (14) is the mass number — protons plus neutrons; the bottom number (6) is the proton number — protons only, which identifies the element [1].

10. 19 electrons: 2 + 8 + 8 = 18, leaving 1 electron in a fourth shell [1]. Configuration: 2,8,8,1 [1]. One outer-shell electron places it in Group I, and four occupied shells place it in Period 4 [1].

11. 2,8 [1]. The sodium atom has lost one electron to form the +1 ion; the number of protons — and therefore the element — stays the same [1].

12. Noble gases have a full outer shell of electrons [1], so they have no tendency to gain, lose or share electrons in a chemical reaction, making them extremely unreactive [1].

13. (0620 Extended, 5070 required) (69 × 60.1) + (71 × 39.9) = 4146.9 + 2832.9 = 6979.8 [1]; 6979.8 ÷ 100 = 69.8 [1].

Examiner insight (June 2025): common errors were to average the two mass numbers without using the percentages (which gives 70.0 here), or to divide the wrong way round. Multiply each mass by its percentage, add, then divide by 100. Try the real question next: Cambridge IGCSE Chemistry 0620, June 2025, Paper 43, Question 2.

---

Where marks are usually lost

• Giving a neutron a relative charge other than 0, or forgetting its relative mass is 1.
• Confusing proton number (identifies the element) with mass number (varies between isotopes).
• Forgetting an ion’s electron count changes but its proton number — and therefore the element — does not.
• Averaging isotope masses without weighting by abundance.
• Saying isotopes have different chemical properties because their mass differs.
• In an “identify the isotope” question, picking an atom with a different proton number (a different element entirely) rather than one with the same proton number but a different mass number/neutron count.
• Writing the mass number and proton number the wrong way round in isotope symbol notation.
• Forgetting that removing an electron to form a positive ion never changes the proton number or the element.