Polyatomic Ions and Ternary Ionic Nomenclature

A polyatomic ion is a group of covalently bonded atoms that carries an overall charge, and a ternary ionic compound is an ionic compound that contains at least three elements because one of its ions is polyatomic; it is named by giving the cation first and the polyatomic anion second, as in calcium carbonate (CaCO₃) and ammonium sulfate ((NH₄)₂SO₄). In brief: memorize the common polyatomic ions, use parentheses around a polyatomic ion when you need more than one of it in the formula, and use the endings -ate and -ite, and the prefixes per- and hypo-, to tell oxyanions apart.

Many common substances, such as baking soda, fertilizers, limestone and household cleaners, are ternary compounds. This article presents the common polyatomic ions with formulas and charges, explains the system of oxyanion names, shows how to write and name compounds with parentheses, works through examples and gives a short history of the naming of oxyanions.

What are the common polyatomic ions?

Common polyatomic ions
Charge	Formula	Name
1+	NH₄⁺	ammonium
1−	OH⁻	hydroxide
1−	NO₃⁻	nitrate
1−	NO₂⁻	nitrite
1−	CN⁻	cyanide
1−	C₂H₃O₂⁻ (CH₃COO⁻)	acetate
1−	HCO₃⁻	hydrogen carbonate (bicarbonate)
1−	HSO₄⁻	hydrogen sulfate (bisulfate)
1−	MnO₄⁻	permanganate
1−	ClO₃⁻	chlorate
2−	SO₄²⁻	sulfate
2−	SO₃²⁻	sulfite
2−	CO₃²⁻	carbonate
2−	CrO₄²⁻	chromate
2−	Cr₂O₇²⁻	dichromate
2−	C₂O₄²⁻	oxalate
2−	S₂O₃²⁻	thiosulfate
2−	O₂²⁻	peroxide
2−	HPO₄²⁻	hydrogen phosphate
3−	PO₄³⁻	phosphate

How are oxyanions named?

Oxyanions are polyatomic ions that contain oxygen and one other element. When an element forms two oxyanions, the one with more oxygen atoms ends in -ate, and the one with fewer ends in -ite: nitrate (NO₃⁻) and nitrite (NO₂⁻), sulfate (SO₄²⁻) and sulfite (SO₃²⁻). When there are four oxyanions, as with chlorine, the prefixes per- (more oxygen than -ate) and hypo- (less oxygen than -ite) are added.

The four oxyanions of chlorine
Formula	Name	Oxidation number of Cl	Oxygen atoms
ClO⁻	hypochlorite	+1	1
ClO₂⁻	chlorite	+3	2
ClO₃⁻	chlorate	+5	3
ClO₄⁻	perchlorate	+7	4

The same pattern applies to bromine and iodine (bromate, iodate, periodate). Adding a hydrogen ion to an oxyanion gives an ion with a lower charge, named with “hydrogen” or the prefix “bi-“: CO₃²⁻ is carbonate, HCO₃⁻ is hydrogen carbonate (or bicarbonate), and PO₄³⁻, HPO₄²⁻ and H₂PO₄⁻ are phosphate, hydrogen phosphate and dihydrogen phosphate.

How do you name a ternary ionic compound?

Name the cation. If it is a metal with variable charge, give the Roman numeral. If it is ammonium, say “ammonium”.
Name the polyatomic anion without changing its ending.
Do not use number prefixes.

How do you write a formula with a polyatomic ion?

Balance the charges as with binary compounds. If more than one polyatomic ion is needed, enclose it in parentheses and put the subscript outside, for example Ca(OH)₂ and Al₂(SO₄)₃. Never use parentheses if only one polyatomic ion is needed: NaOH, not Na(OH). Never change the subscripts inside the polyatomic ion.

Examples of ternary compounds

Ternary ionic compounds
Formula	Ions	Name	Common use
CaCO₃	Ca²⁺, CO₃²⁻	calcium carbonate	Limestone, chalk
NaHCO₃	Na⁺, HCO₃⁻	sodium hydrogen carbonate	Baking soda
(NH₄)₂SO₄	2NH₄⁺, SO₄²⁻	ammonium sulfate	Fertilizer
Ca₃(PO₄)₂	3Ca²⁺, 2PO₄³⁻	calcium phosphate	Bones, fertilizer
KMnO₄	K⁺, MnO₄⁻	potassium permanganate	Disinfectant, oxidizing agent
Fe(NO₃)₃	Fe³⁺, 3NO₃⁻	iron(III) nitrate	Etching, laboratory use
Mg(OH)₂	Mg²⁺, 2OH⁻	magnesium hydroxide	Antacid (milk of magnesia)

Worked examples

Name Cu(NO₃)₂. Two nitrate ions (NO₃⁻) carry −2, so copper is +2. The name is copper(II) nitrate.
Formula of aluminum sulfate. Al³⁺ and SO₄²⁻. The crisscross gives Al₂(SO₄)₃. Check: 2 × 3 = +6 and 3 × 2 = −6.
Formula of ammonium phosphate. NH₄⁺ and PO₄³⁻. Three ammonium ions are needed: (NH₄)₃PO₄.
Name Fe₂(SO₄)₃. Three sulfate ions carry −6, so two iron ions carry +6 and each is +3. The name is iron(III) sulfate.
Count atoms. In (NH₄)₂SO₄: N = 2, H = 2 × 4 = 8, S = 1, O = 4, so there are 2 + 8 + 1 + 4 = 15 atoms.
Molar mass. (NH₄)₂SO₄: 2 × 14.01 + 8 × 1.008 + 32.07 + 4 × 16.00 = 28.02 + 8.064 + 32.07 + 64.00 = 132.15 g/mol. The mass percent of nitrogen is 28.02 ÷ 132.15 × 100% = 21.2%, which is why it is valued as a nitrogen fertilizer.

How can you learn the ions?

Learn the “-ate” ions first: nitrate NO₃⁻, sulfate SO₄²⁻, phosphate PO₄³⁻, carbonate CO₃²⁻, chlorate ClO₃⁻. The “-ite” ion has one oxygen less with the same charge.
Notice patterns: the charge of the “-ate” ions of the same family, such as nitrate and carbonate, depends on the position of the central atom in the periodic table.
Use flashcards, and practice writing formulas both ways.

Where do these names come from?

The -ate and -ite endings were proposed in the 1787 system of Lavoisier and his colleagues: acids with the ending “-ic” gave salts ending in “-ate”, and acids with the ending “-ous” gave salts ending in “-ite”. The prefixes per- and hypo- were added in the early 19th century when chemists found more oxygen-containing acids of the same element. The system is still the basis of the IUPAC names, although IUPAC also recommends names that show the number of oxygen atoms, such as “tetraoxidosulfate(2−)”, which are used rarely in teaching.

Which mistakes are common?

Mistake 1: Using parentheses for one polyatomic ion

Write NaOH, not Na(OH). Parentheses are needed only for subscripts greater than 1.

Mistake 2: Changing the subscripts inside the ion

Sulfate is always SO₄²⁻. To get more oxygen, use more ions, not different subscripts.

Mistake 3: Mixing up -ate and -ite

The -ate ion has more oxygen. Nitrate is NO₃⁻, and nitrite is NO₂⁻.

Mistake 4: Forgetting the charge of ammonium

Ammonium is a cation, NH₄⁺, and ammonia, NH₃, is a neutral molecule.