1)Some polyatomic anions are formed by the attachment of one or more hydrogen atoms. In fact, it is common for hydrogen atoms to be transferred from one ion or molecule to another ion or molecule. When this happens, the hydrogen atom is usually transferred without its electron, as H+. If an anion has a charge of -2 or -3, it can gain one or two H+ ions and still retain a negative charge. For example, carbonate, CO32-, can gain an H+ ion to form HCO3-, which is found in baking soda. The sulfide ion, S2-, can gain one H+ ion to form HS-. Phosphate, PO43-, can gain one H+ ion and form HPO42-, or it can gain two H+ ions to form H2PO4-. These polyatomic ions are named with the word hydrogen in front of the name of the anion if there is one H+ ion attached and dihydrogen in front of the name of the anion if two H+ ions are attached.
HCO3- is hydrogen carbonate ion.
HS- is hydrogen sulfide ion.
HPO42- is hydrogen phosphate ion.
H2PO4- is dihydrogen phosphate ion.
Some polyatomic ions also have nonsystematic names that are often used. For example, HCO3– is often called bicarbonate instead of hydrogen carbonate.
2) Binary ionic compounds
Ionic compounds whose formula contains one symbol for a metal and one symbol for a nonmetal are called binary ionic compounds.
For example,the general formula is MaAb, “M” represents the symbol of a metallic element, “A” represents the symbol for a nonmetallic element, and lowercase “a” and “b” represents the subscripts in the formula (unless one or more of the subscripts are assumed to be 1).
3)Predicting Monatomic Anion Charges
The fact that the noble gas atoms do not gain, lose, or share their electrons suggests there must be something especially stable about having 2 (helium, He), 10 (neon, Ne), 18 (argon, Ar), 36 (krypton, Kr), 54 (xenon, Xe), or 86 (radon, Rn) electrons.Stability is reflected in the fact that nonmetallic atoms form anions in order to get the same number of electrons as the nearest noble gas.
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