Question

Is calcium cation a hard acid or soft acid?

Answer 1

It should have fairly decent polarizing ability... it has a common charge of `+2`, but on the other hand, it is a large cation relative to the other alkaline earth metals. The high charge contributes to a tighter/harder electron cloud, but the large size counteracts that to some extent.

This makes it a soft-ish, maybe medium-soft acid according to "Hard Soft Acid Base Theory". What that means is that the electron density is not very tightly packed, and thus, it is not readily pulled towards it, and it is not a very polarizing cation in the presence of anions.

Sodium, however, is a hard acid. It is rather small, and thus polarizes electron density towards it well (not to be confused with its behavior based on electronegativity!). All that means is that its electron density is tightly packed; it does not mean sodium metal is highly electronegative; it means that sodium cation is highly electropositive.