Metallic atoms hold some of their electrons relatively loosely, and as a result, they tend to lose electrons and form cations. In contrast, nonmetallic atoms attract electrons more strongly than metallic atoms, and so nonmetals tend to gain electrons and form anions. Thus, when a metallic element and a nonmetallic element combine, the nonmetallic atoms often pull one or more electrons far enough away from the metallic atoms to form ions. The positive cations and the negative anions then attract each other to form ionic bonds.
The atoms of the noble gases found in nature are uncombined with other atoms. 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. This 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.
The image below summarizes the charges of the ions that you should know at this stage.
The monatomic anions are named by adding -ide to the root of the name of the nonmetal that forms the anion. For example, N3- is the nitride ion. The names of the anions are below .
hydride ion, H-
nitride ion, N3-
phosphide ion, P3-
oxide ion, O2-
sulfide ion, S2-
selenide ion, Se2-
fluoride ion, F-
chloride ion, Cl-
bromide ion, Br-
iodide ion, I-
There is many polyatomic anions. The following anions are most common.
hydroxide ion, OH-
nitrate ion, NO3-
acetate ion, C2H3O2-
carbonate ion, CO32-
sulfate ion, SO42-
phosphate ion, PO43-
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. You should avoid using this less accepted name, but because many people still use it, you should know it.