Use data from the Data Booklet to predict the geometry of, and therefore the co-ordination number of, the cation in sodium chloride, $\text{NaCl}$, and magnesium chloride, $\text{MgCl}_2$.
Magnesium(I) chloride, $\text{MgCl}$, is an unstable compound and decomposes readily as shown: $\text{2MgCl}(s) \rightarrow \text{Mg}(s) + \text{MgCl}_2(s)$. Use the data below to calculate the enthalpy change of this reaction: $\Delta H_f^{\circ}(\text{MgCl}(s)) = -106\ \text{kJ mol}^{-1}$ and $\Delta H_f^{\circ}(\text{MgCl}_2(s)) = -642\ \text{kJ mol}^{-1}$.
The equation that defines $\Delta H$ as the lattice energy for $\text{MgCl}$ is given: $\text{Mg}^+(g) + \text{Cl}^-(g) \rightarrow \text{MgCl}(s)$. Use the equation, the following data, and relevant data from the Data Booklet to calculate a value for the lattice energy of $\text{MgCl}$. Electron affinity of $\text{Cl}(g) = -349\ \text{kJ mol}^{-1}$; enthalpy change of atomisation of $\text{Mg}(s) = +147\ \text{kJ mol}^{-1}$; enthalpy change of formation of $\text{MgCl}(s) = -106\ \text{kJ mol}^{-1}$.
Suggest how the lattice energies of $\text{MgCl}_2$ and $\text{NaCl}$ will compare with that of $\text{MgCl}$. Explain your answers.
Define electron affinity.