An explanation that makes reference to the following points:
- on descending Group 2, lattice enthalpy becomes less exothermic / less negative / less energy is released (1)
- as the radius/size of the metal ion / M^2+ ion / cation increases while the charge on the ions remains the same (1)
- there are weaker forces of attraction between the ions / less attraction to chloride ions (1)
Award reverse arguments if clearly referring to ascending the group. Penalise mention specifically of atoms instead of ions once only; penalise mention specifically of molecules once only. Ignore discussion of polarising power.
