The Born–Haber cycle for the formation of magnesium oxide, MgO, is shown using the following enthalpy changes (all values in kJ mol⁻¹):
Enthalpy of formation of MgO: –602 Enthalpy of atomisation of Mg: +148 First ionisation energy of Mg: +736 Second ionisation energy of Mg: +1450 Enthalpy of atomisation of O: +249 First electron affinity of O: –141 Second electron affinity of O: +798 Lattice energy of MgO: –3889
Use these data and your knowledge of Born–Haber cycles and lattice energy to answer the following.
(a) Describe what is meant by the term 'enthalpy of atomisation' as applied to magnesium and to oxygen.
(b) Explain why the first electron affinity of oxygen is exothermic but the second electron affinity is endothermic.
(c) State and explain how the magnitude of the lattice energy of MgO compares with that of CaO and with that of NaCl.