CBSE Class 9 Answered

Negative 2 charge is the result of the one or two elements that were originally bonded to carbonate ion. The 2- indicates the electrostatic charge on the ion as a result of gaining electrons (Positive charge indicates loss of electrons). Metals form ions with positive charges. Nonmetals and polyatomic ions generally have negative charges.
Consider the case of Carbonic acid, H₂CO₃. When it is dissolved in water, water molecules breaks the bonds between Hydrogen and oxygen. But, since the oxygen is much more electronegative than hydrogen, bonding pair of electrons was drawn closer to oxygen. Oxygen keeps the bonding pair of electrons giving it a net gain of one electron from one hydrogen atom. When this is repeated with the other hydrogen atom the resulting carbonate ion has gained two electrons, giving it a charge of 2-.