Marking Scheme — Isotopes and Ionic Bonds Practice
1. Define isotopes; examples of carbon isotopes [3]
- Definition: Atoms of the same element with the same number of protons but different numbers of neutrons. (1)
- Example 1: 12C → p = 6, n = 6. (1)
- Example 2: 13C → p = 6, n = 7. (1)
Accept any correct carbon isotopes (e.g., 14C: p=6, n=8) with correct proton/neutron counts.
2. Why same chemical but different physical properties? [3]
- Chemical properties: Depend on electron configuration; isotopes have identical electron structures (same protons/electrons). (2)
- Physical differences: Different masses affect density, rates of diffusion, melting/boiling points (slightly), etc. (1)
3. Interpret symbols [4]
- 2311Na (neutral): p = 11 (1), n = 12 (1), e = 11 (1)
- 168O2−: p = 8 (0.5), n = 8 (0.5), e = 10 (1)
4. Relative atomic mass of chlorine [3]
RAM = (35 × 75.77 + 37 × 24.23) / 100
= (2651.95 + 896.51) / 100
= 3548.46 / 100
= 35.4846 ≈ 35.5
Award (1) for correct method (weighted average), (1) for correct substitution, (1) for answer to 1 d.p. (35.5).
5. MCQ — NOT true about ionic bonds [1]
- Answer: B) They involve sharing of electrons (that is covalent). (1)
6. Formation of Ca2+ and F− [4]
- Calcium: Ca → Ca2+ + 2e− (loss of two 4s electrons). (2)
- Fluoride: F + e− → F− (gain of one electron to complete octet). (2)
7. Dot-and-cross for MgO [4]
- Shows transfer of 2 electrons from Mg to O. (1)
- Oxygen shown with a full octet (8 e−). (1)
- Both ions in square brackets with charges: [Mg]2+, [O]2−. (1)
- Distinct symbols (dots/crosses) used for Mg vs O original electrons. (1)
Penalise only once for missing brackets/charges. Allow any clear equivalent representation.
8. Conductivity of NaCl (solid vs molten) [3]
- Solid: ions are fixed in a giant lattice; no mobile charge carriers. (1–2)
- Molten: lattice breaks; ions are free to move and carry charge. (1)
9. MCQ — Giant ionic lattice arrangement [1]
- Answer: D) All of the above. (Each + ion surrounded by − ions, each − by +, regular repeating pattern.) (1)
10. Relative atomic mass of copper [3]
RAM = (63 × 69.17 + 65 × 30.83) / 100
= (4358. + 2003.95) / 100
= 6361.95 / 100
= 63.6165 ≈ 63.6
Award (1) method, (1) substitution, (1) correct rounding (e.g., 63.6 or 63.62).
11. Compare ionic vs covalent compounds [6]
- Melting/boiling points: Ionic typically high (strong electrostatic attractions); covalent (simple molecules) typically low (weak intermolecular forces). (2) Accept exception: giant covalent (e.g., diamond, SiO₂) have very high m.p. (credit as additional detail).
- Electrical conductivity: Ionic conduct when molten or in aqueous solution (mobile ions), not when solid; covalent generally do not conduct (no ions/charge carriers), except graphite. (2)
- Physical state at room temp: Ionic usually crystalline solids; covalent often gases/liquids/low-m.p. solids. (2)
12. Relative atomic mass of boron ≈ 10.8 [3]
RAM = (10 × 19.9 + 11 × 80.1) / 100
= (199 + 881.1) / 100
= 1080.1 / 100
= 10.801 ≈ 10.8
Award (1) method, (1) correct working, (1) stated ≈ 10.8.
13. MCQ — Pair forming an ionic compound [1]
- Answer: B) Sodium and chlorine (metal + non-metal; electron transfer). (1)
14. Properties of potassium iodide (KI) [4]
- High m.p. (681 °C): Strong electrostatic attraction between K+ and I− in a giant ionic lattice requires large energy to overcome. (2)
- Brittleness (solid): Under stress, layers shift so like charges align (K+ near K+, I− near I−) causing strong repulsion and cleavage. (2)
15. Relative atomic mass of element X [3]
RAM = (28 × 92.2 + 29 × 4.7 + 30 × 3.1) / 100
= (2581.6 + 136.3 + 93.0) / 100
= 2810.9 / 100
= 28.1 (to 1 d.p.)
Award (1) method, (1) correct working, (1) rounded answer 28.1.
