The arrangement of the anions around a cation is called the geometry of the cation; e.g. in [CuCl4]2- the geometry of copper is tetrahedral and the co-ordination number of copper is 4. The geometry of a cation in an ionic compound can be predicted from the ratio of the ionic radii of the cation and anion involved. [Table 5.1: Cation radius/anion radius vs. geometry of cation] Use data from the Data Booklet to predict the geometry of, and hence the co-ordination number of, the cation for

1. (a)(i)• sodium chloride, NaCl, geometry of Na⁺ = ................................. co-ordination number of Na⁺ = ................................[2]
2. (a)(ii)• magnesium chloride, MgCl2. geometry of Mg²⁺ = ................................ co-ordination number of Mg²⁺ = ................................[2]
3. (b)Magnesium(I) chloride, MgCl, is an unstable compound and readily decomposes as shown. 2MgCl(s) → Mg(s) + MgCl2(s) Use the following data to calculate the enthalpy change of this reaction. ΔHf MgCl(s) = −106 kJ mol⁻¹ ΔHf MgCl2(s) = −642 kJ mol⁻¹[1]
4. (c)(i)The equation for which ∆H is the lattice energy for MgCl is shown. Mg⁺(g) + Cl⁻(g) → MgCl(s) Use the equation, the following data, and relevant data from the Data Booklet to calculate a value for the lattice energy of MgCl. You might find it helpful to construct an energy cycle. electron affinity of Cl(g) = -349 kJ mol⁻¹ enthalpy change of atomisation of Mg(s) = +147 kJ mol⁻¹ enthalpy change of formation of MgCl(s) = −106 kJ mol⁻¹[3]
5. (c)(ii)Suggest how the lattice energies of MgCl2 and NaCl will compare to that of MgCl. Explain your answers.[3]
6. (d)Define the term electron affinity.[2]