What does mg2co3 mean?
I believe it is Magnesium Oxide. The 2 should be subscript,
denoting the presence of two atoms of magnesium. This is because
magnesium has a charge of -1, versus oxygen's typical charge of +2.
Two atoms of magnesium balances the compound. No, magnesium oxide
would be MgO - the magnesium has an oxidation number (or charge) of
+2, compared with -2 for oxygen, so one of each is needed to
balance out the molecule. I don't think Mg2O exists.