# 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?

Cu2+ has an incomplete 3d orbital (4s0 3d9) and can therefore be included in the chemistry of transition metals. As it can be coordinated (form dative covalent bonds) with ligands to create complex ions, for example H20 in hexaaquacopper(ii), it acts as a Lewis acid.

But in terms of Brønsted–Lowry theory, perhaps the textbook is referring to the fact that when transition metals are dissolved in water they can donate an H+ ion to another H2O molecule to form H3O+. This can be observed with Fe3+:

[Fe(H20)6]3+ + H2O -> [Fe(H20)5(OH)]2+ + H3O+

The complex ion is acting as an acid by donating a hydrogen ion to water molecules in the solution. The water is, of course, acting as a base by accepting the hydrogen ion.

• Well-explained! So, I'm assuming calcium doesn't form an aqueous complex, even though it is positively charged? – Mew Jan 3 at 17:41
• @Mew Ca does form aquo complexes, they are much weaker acids though. – Mithoron Jan 3 at 18:50