Anion Names and Formulas
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
Predicting Monatomic Anion Charges
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.
Monatomic Anion Names
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
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
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.