HgCl is the so-called "empirical" formula, not the structural formala, and many old chemistry textbooks give HgCl. But modern techniques of crystal analysis show that the metallic ion is actually two mercury atoms bonded together as a single unit and giving up two electrons between them. The mercurous ion, also called the mercury (I) ion, is in reality Hg2++ rather than Hg+ as earlier believed.

The chloride ion in either case is simply a chlorine atom which has captured a single electron, shown as Cl-. The crystal lattice for mercurous chloride has an Hg2++ ion in the center and a Cl- ion at either side. The crystal lattice for mercuric chloride, also called mercury (II) chloride, has an Hg++ ion in the center and a Cl- ion at either side.

Hg2Cl2 is now the accepted formula for mercurous chloride. HgCl2 is and has always been the formula for mercuric chloride.