Question

Why is the nomenclature for Mercury (I) Fluoride Hg2F2 and not HgF?

Answer 1

Because mercurous ion is the `Hg_2^(2+)` ion...........The empirical formula of mercurous halide may be `HgX`, but its molecular formula is `Hg_2X_2`.

Explanation:

And thus `"mercurous halide"` is `Hg_2X_2`; and `"mercuric halide"` is `HgX_2`. As is common with `"ous"` versus `"ic"` endings, the `"ic"` denotes the higher oxidation state.

In `HgX_2` the metal displays a formal `+II` oxidation state; in `Hg_2X_2` the metal displays a formal `+I` oxidation state. Where there is an `"element-element bond"`, as here in `Hg-Hg`, or in a `C-C` linkage, the 2 electrons of the bond are assumed to be distributed EQUALLY to the bound atoms. When we assign oxidation states for `"element-heteroelement bonds"`, the most electronegative atom gets the 2 electrons from the bond.

And thus for `HO-CR_3`, we get formally `HO^(-)` and `""^(+)CR_3`. The ipso carbon in `""^(+)CR_3` has a FORMAL `C(+I)` oxidation state.

I write `"formal"` guardedly. Of course with such designations, the assignment of oxidation number is very much a formalism, a practice with little fundamental significance, but there is utility in assigning oxidation numbers for balancing redox reactions.