Explain why the enthalpy of lattice dissociation of potassium oxide is less endothermic than that of sodium oxide. ( 2 Marks)

It is important with any exam question to decide what you think is required to achieve each individual mark. In this question, I'd read it and decide that the two marks are for: 1) Describing the relevant difference between the two compounds 2) Stating why the cause the difference in lattice dissociation enthalpy. It is obvious from the question that the major difference lies in the singly positively charged ion of the lattice - Potassium or Sodium. As they both have a single positive charge, the difference is in the ionic radius and therefore charge density - potassium is larger and therefore has a lower charge density. Therefore Potassium ions have a weaker attraction to negatively charged oxide ions within the lattice - leading to a less endothermic dissociation enthalpy.