Naming ionic compounds containing polyatomic ions

Definition

Polyatomic ions are ions which consist of more than one atom. For example, nitrate ion, NO3-, contains one nitrogen atom and three oxygen atoms.

Rules

1. The cation is written first in the name; the anion is written second in the name.

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.

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 ammonium chloride is "NH4Cl" not " (NH4)Cl ").

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., Mg2+ = "magnesium"). 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., Cu+ = "copper (I)").

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").

Note: Greek prefixes are not used to indicate the number of atoms, or polyatomic ions, in the formula unit for the compound (e.g., Ba(ClO3)2 is named "barium chlorate" not "barium dichlorate").

C₂H₃O₂⁻

Ca(CN)₂

Calcium cyanide

KNO₂

Potassium nitrite

Ba(NO₃)₂

Barium nitrate

Cd₃(PO₄)₂

Cadmium phosphate

Co(CH₃CO₂)₃

Cobalt (III) acetate

Ca(MnO₄)₂

Calcium permanganate

KOH

Potassium hydroxide

BaCr₂O₇

Barium dichromate

Li₂HPO₄

Lithium hydrogen phosphate

Na₂SO₄

Sodium sulfate

Mg₃(PO₄)₂

Magnesium phosphate

Ba(OH)₂

Barium hydroxide

MgSO₃

Magnesium sulfite

Na₃PO₄

Sodium phosphate

RbOH

Rubidium hydroxide

Ti(NO₃)₂

Titanium (II) nitrate

Sn(OH)₂

Tin (II) hydroxide

NiPO₄

Nickel (III) phosphate