POLYATOMIC IONS
NAME
ammonium
acetate
bromate
chlorate
chlorite
cyanide
dihydrogen phosphate
hypochlorite
hydrogen carbonate
(bicarbonate)
hydrogen Sulfate (bisulfate)
hydrogen Sulfite (bisulfite)
hydroxide
iodate
nitrate
nitrite
perchlorate
permanganate
thiocyanate
carbonate
chromate
dichromate
oxalate
selenate
silicate
sulfate
sulfite
phosphate
phosphite
SYMBOL/CHARGE
NH4+
C2H3O2- or CH3COOBrO3ClO3ClO2CNH2PO4ClOHCO3HSO4HSO3OHIO3NO3NO2ClO4MnO4SCNCO32CrO42Cr2O72C2O42SeO42SiO32SO42SO32PO43PO33-
Rules for Naming and Writing Formulas for Ionic Compounds
Containing Polyatomic Ions
Polyatomic ions are ions which consist of more than one atom. For example,
nitrate ion, NO3-, contains one nitrogen atom and three oxygen atoms. The atoms
in a polyatomic ion are usually covalently bonded to one another, and therefore
stay together as a single, charged unit.
Naming and writing formulas follow the same rules as binary ionic compounds.
However, the most important part is recognizing a polyatomic ion is present.
How does a person recognize that a polyatomic ion is present? In binary
compounds only two different elements are present in the bond. In compounds
containing polyatomic ions there are three or more different elements present.
Keep in mind that even in compounds containing polyatomic ions, there are still
only two ions present (one cation and one anion).
Notice that there are a lot more polyatomic anions than cations. Most polyatomic
anions, consist of a nonmetallic element combined with different numbers of
oxygen atoms (these polyatomic anions are called oxoanions.) Even though it
seems there is no simple rule in naming these ions, here are some guidelines to
follow:
Rule 1. The cation is written first in the name; the anion is written second in the
name.
Rule 2. When the formula contains two or more of the same polyatomic ion, that
ion is written in parentheses with the subscript written outside the parentheses.
Note: parentheses and a subscript are not used unless more than
one of a polyatomic ion is present in the formula unit (e.g., the
formula unit for calcium sulfate is "CaSO4" not "Ca(SO4)").
Rule 3. If the anion is a monatomic ion, the anion is named by adding the suffix ide to the root of the element name (e.g., I- = "iodide").
Note: Greek prefixes are not used to indicate the number of atoms, or polyatomic
ions, in the formula unit for the compound (e.g., Ca(NO3)2 is named "calcium
nitrate" not "calciuim dinitrate").