Atomic Structure: 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 revision notes

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

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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]

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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].

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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.