chem問題

All Group I metals have giant metallic structure, in which the metal atoms are held together by metallic bonds.

The strength of metallic bond depends on two factors :
(1) The greater the number of delocalized electrons per atom, the stronger is the metallic bond.
(2) The smaller the atomic size, the stronger is the metallic bond. This is the delocalized electrons are more strongly attracted by the nuclear charge for a smaller atom.

Since each Group I metal has only 1 delocalized electron per atom, we consider factor (2) only.
Down Group I, the atomic sizes are increasing as the increase in number of shells. Therefore, down Group I, the strength of metallic bonds is decreasing, and thus the melting points are decreasing.