Mercury in normal conditions has oxidation states 1 and 2. Thus it can only form Mercury (I) Oxide and Mercury (II) Oxide. Although there had been a very famous propoganda in the 1970s that people had systhesized mercury (III) oxide under very special conditions, but as there were no proofs or developments after the declaration, it is now generally believed to have been a false claim. Recently, in 2007, Mercury (IV) Fluoride was created, but Mercury (III) Oxide is not present. The electronic configuration of mercury helps give visual understanding to this concept.
HgO is the formula for mercury ii oxide.
Mercury only has a two and one oxidation state. HgO would be the formula for mercury II oxide.
there is no formula for mercury iii oxide, but there is one for mercury ii oxide