The following are the lattice energies of Group 1A compounds: LiH, 916 kJ/mol; NaH, 807 kJ/mol; KH, 711 kJ/mol; RbH, 686 kJ/mol; CsH, 648 kJ/mol. 

CuH has a lattice energy of 828 kJ/mol. Identify the Group 1A element that is most similar in ionic radius to the Cu⁺ ion considering the trend in lattice enthalpies in the Group 1A compounds.