Polyatomic Ions Chart (Page 2 of 2) in pdf

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, NO

, 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 "CaSO

" not "Ca(SO

)").

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(NO

)

is named "calcium

nitrate" not "calciuim dinitrate").

Polyatomic Ions Chart Page 2