# Acidic tendency of copper(II) ion vs. calcium(II) ion

In my Chemistry textbook, a table is shown wherein ions originating in various salts are categorised according to their acidic or basic tendencies. Among others, $$\ce{Cu^2+}$$ and $$\ce{Ca^2+}$$ are present in the table; however, the copper ion is classified as being acidic, whilst the calcium ion is classified as being neutral.

I don't really see how they differ, apart from the amount of electrons they have in non-valence shells and the subshell that holds their valence electrons. Either way, since this is in the context of Brønsted–Lowry and not Lewis, I have no clue how the textbook got to its conclusion; does it have to do with copper parttaking in any redox reactions, or perhaps a certain stability calcium has?

• It's still Lewis needed to analyse this - it's acidity of metal aquo complexes. – Mithoron Jan 3 '19 at 18:46