Mercurous ion is written as `Hg_(2)^(2+)` whereas cuprous ion is written as `Cu^(+)` . Explain.

Explanation

The electronic configuration of Hg(I), i.e., `Hg^(+) ` is `[Xe] 4f^(14)5d^(10) 6s^(1)` and thus has one electron in the valence 6s-orbital. If this were so, all Hg(I) compounds should be paramagnetic butactually they are diamagnetic. This behaviour can be explained if we assume that the singly filled 6s-orbitals of the two `Hg^(+)` ions overlap to form a Hg-Hg covalent bond. Thus, `Hg^(+)` ions exist as dimeric species, i.e., `Hg_(2)^(2+)` . In contrast, the electronic configuration of Cu(I) ion, i.e., `Cu^(+)` is `[Ar] 3d^(10)` . Therefore, it has no unpaired electrons to form dimeric species, i.e., `Cu_(2)^(2+)` and hence it always exists as `Cu^(+)` ion.