Explain, why mercury (I) ion exists as `Hg_(2)^(2+)` ions, while copper (I) exists as `Cu^(+)` ion.

Explanation

The electronic configuration of Hg (I) is `[Xe]4f^(14)5d^(10)6s^(1)`. It has one unpaired electron in the valence `6s`-subshell. It is paramagnetic but actually Hg(I) compounds are diamagnetic. This change can be explained by assuming that the singly filled `6s`-orbitals of two `Hg^(+)` ion overlap to from Hg-Hg covalent bond.
Therefore, `Hg^(+)` ion exists as dimeric species, i.e. `Hg_(2)^(2+)`. On the other hand, Cu(I) ion has electronic configuration `[Ar]3d^(10)`. It has no unpaired electron to form dimeric `Cu_(2)^(2+)` species and therefore, its exists as `Cu^(+)` ion.