Here's why:

* Mercury (Hg) has two common oxidation states: +1 and +2.

* Mercurous refers to the +1 oxidation state of mercury.

* Chloride is the anion formed from chlorine (Cl), with a -1 charge.

Therefore, to balance the charges, two mercury ions (+1 each) combine with two chloride ions (-1 each), giving the formula Hg₂Cl₂.