(i) «electronegativity difference = 3.2 − 1.3 =» 1.9 AND «average electronegativity = (3.2 + 1.3)/2 =» 2.25 / 2.3 ✔

ionic AND «approximately» 40 % covalent ✔

(ii) three values from the data booklet: «Cl–Cl bond enthalpy» 242 AND «first ionization energy of Mg» 738 AND «electron affinity of Cl» −349 «kJ mol⁻¹» ✔

«2 × (−349) =» −698 «kJ mol⁻¹» ✔

641 + 148 + 738 + 1451 + 242 = $\Delta H_\text{latt}$ + 698 AND «$\Delta H_\text{latt}$ = 3220 − 698 = +2522 =» +2520 «kJ mol⁻¹» ✔

(iii) $\ce{Na+}$ and $\ce{Mg^2+}$ have lost their 3s electrons / have electrons in only 2 shells / energy levels «[Ne], so both are smaller than $\ce{Cl-}$

$\ce{Mg^2+}$ has a greater nuclear charge / more protons than $\ce{Na+}$ «12 compared with 11, so the same electrons are pulled closer: 72 pm compared with 102 pm» ✔

$\ce{Cl-}$ has electrons in 3 shells / energy levels / 3p⁶ «so it is the largest, 181 pm» ✔

(iv) magnesium chloride AND «$\ce{Mg^2+}$ has a» smaller radius / higher charge density «than $\ce{Ca^2+}$» ✔

(i) M1: both electronegativity quantities from section 9; M2: ionic with a percentage covalent character in the range 35–45 %. (ii) M1: the three booklet values; M2: the electron affinity doubled; M3: correct application of Hess's law and the lattice enthalpy, +2520 kJ mol-1. Award [3] for the correct final answer; award [2 max] for −2520 kJ mol-1. M1 and M2 may be scored on the diagram. ECF for M3 from the candidate's own values. (iii) [1] for each point. (iv) [1] for magnesium chloride with the reason.