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

Explanation

`Hg(Z=80)implies4f^(14)5d^(10)6s^2,Hg^(o+)implies4f^(14)5d^(10)6s^1`
`Hg^(o+)` has one electron in its valence 6s-orbital, due to this `Hg^(o+)` compounds should be paramagenetic but actually they are diamagnetic. Hence the singly filled 6s-orbitals of the two `Hg^(o+)` ions overlap to form a Hg-Hg covalent bond. Thus `Hg^(o+)` ions exist as dimetic species i.e., `Hg_2^(2+)`, `Cu(Z=29)implies3d^(10)4s^1`
`Cu^(o+)implies3d^(10)`
Therefire, `Cu^(o+)` ion has no unpaired electrons to form dimeric species i.e., `Cu_2^(2+)` and hence it always exists as `Cu^(o+)` ion.