Polyatomic Ions- are molecules made up of 2 or more atoms that are considered an ionic group, that is, a molecule with a charge.
Rule 1. The cation is written first in the name; the anion is written second in the name.
Rule 2. When the formula unit contains two or more of the same polyatomic ion, that ion is written in parentheses with the subscript written outside the parentheses.
Rule 3. If the cation is a metal ion with a fixed charge, the name of the cation is the same as the (neutral) element from which it is derived (e.g., Na+ = "sodium"). If the cation is a metal ion with a variable charge, the charge on the cation is indicated using a Roman numeral, in parentheses, immediately following the name of the cation (e.g., Fe3+ = "iron(III)").
Rule 4. 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").