Naming Oxyanions: -ate, -ite, per- and hypo-

Nitrate and nitrite, sulfate and sulfite, perchlorate and hypochlorite — oxyanion names can look like a jumble of similar words. But they follow a tidy system based on how many oxygen atoms the ion contains relative to a reference ion. Once you know the system, you can name unfamiliar oxyanions, predict formulas from names, and connect them directly to acid names. This guide explains the rules and the logic behind them.

What is an oxyanion?

An oxyanion is a negative polyatomic ion containing oxygen and one other element (sometimes with hydrogen too). The central atom is covalently bonded to oxygen atoms, and the whole group carries a negative charge. Examples: NO₃⁻, SO₄²⁻, PO₄³⁻, ClO⁻, CO₃²⁻.

Many elements form more than one oxyanion, with different numbers of oxygen atoms. The naming system tells them apart.

The basic rule: -ate and -ite

For an element that forms two common oxyanions:

• The one with more oxygen atoms ends in -ate.
• The one with one fewer oxygen atom ends in -ite.
• Both have the same charge.

Element	-ate ion	-ite ion
Nitrogen	nitrate, NO₃⁻	nitrite, NO₂⁻
Sulfur	sulfate, SO₄²⁻	sulfite, SO₃²⁻
Phosphorus	phosphate, PO₄³⁻	phosphite, PO₃³⁻

Important: “-ate” doesn’t mean a particular number of oxygens. Nitrate has three oxygens; sulfate has four. The endings only compare ions of the same element. So you do need to learn one reference ion for each element — usually the -ate ion — and work out the rest from it.

Four oxyanions: per- and hypo-

The halogens chlorine, bromine and iodine each form four oxyanions. Two prefixes extend the series:

• per- + -ate: one more oxygen than the -ate ion.
• hypo- + -ite: one fewer oxygen than the -ite ion.

The chlorine series:

Name	Formula	Oxygens	Oxidation state of Cl
perchlorate	ClO₄⁻	4	+7
chlorate	ClO₃⁻	3	+5
chlorite	ClO₂⁻	2	+3
hypochlorite	ClO⁻	1	+1

All four have a charge of −1. The same pattern gives bromate (BrO₃⁻), hypobromite (BrO⁻), iodate (IO₃⁻), periodate (IO₄⁻), and so on. (“Periodate” is pronounced per-iodate — nothing to do with the periodic table!)

A memory aid

Think of the series as a ladder, from most oxygen to least:

per…ate → …ate → …ite → hypo…ite

“Per” means “beyond” or “more”; “hypo” means “under” or “less” (as in hypothermia).

Oxidation states: what the names really track

The naming system is really tracking the oxidation state of the central atom (see how to find oxidation numbers). In the chlorine series, each extra oxygen raises chlorine’s oxidation state by 2 (from +1 to +7), while the overall charge stays the same.

You can calculate the oxidation state quickly. For sulfate, SO₄²⁻:

• Each O is −2 → 4 × (−2) = −8
• Overall charge is −2 → S + (−8) = −2 → S = +6

For sulfite, SO₃²⁻: S + 3(−2) = −2 → S = +4.

Higher oxidation states generally mean stronger oxidising agents: perchlorates and chlorates are powerful oxidisers, and must be handled with care. You can check oxidation states with the oxidation number calculator.

Patterns across the periodic table

The common -ate ions of the period 2 and period 3 elements show useful patterns:

Group	14	15	16	17
Period 2	carbonate CO₃²⁻	nitrate NO₃⁻	—	—
Period 3	silicate SiO₄⁴⁻	phosphate PO₄³⁻	sulfate SO₄²⁻	perchlorate ClO₄⁻ (chlorate ClO₃⁻)

• Period 2 -ate ions have three oxygens: CO₃²⁻, NO₃⁻.
• Period 3 ions have four oxygens: SiO₄⁴⁻, PO₄³⁻, SO₄²⁻, ClO₄⁻.
• Across period 3, the charge goes −4, −3, −2, −1 as the central atom’s group increases.

Smaller period 2 atoms can fit only three oxygens around them and form double bonds more readily, while larger period 3 atoms fit four.

Hydrogen-containing oxyanions

Adding H⁺ to an oxyanion reduces the negative charge by 1, and the name adds “hydrogen” (or dihydrogen) in front:

Parent ion	+ H⁺	+ 2H⁺
carbonate CO₃²⁻	hydrogencarbonate HCO₃⁻	(carbonic acid H₂CO₃)
sulfate SO₄²⁻	hydrogensulfate HSO₄⁻	(sulfuric acid H₂SO₄)
phosphate PO₄³⁻	hydrogenphosphate HPO₄²⁻	dihydrogenphosphate H₂PO₄⁻

Older names use bi-: bicarbonate (HCO₃⁻) and bisulfate (HSO₄⁻). “Bicarbonate of soda” is sodium hydrogencarbonate, NaHCO₃. The hydrogencarbonate ion is crucial in the body’s main blood buffer (see the blood buffer system).

Other prefixes you might meet

• thio- means one oxygen replaced by sulfur: sulfate SO₄²⁻ → thiosulfate S₂O₃²⁻ (used in photography and in the iodine–thiosulfate titration).
• di- (or pyro-) means two central atoms sharing an oxygen: dichromate Cr₂O₇²⁻ (from chromate CrO₄²⁻), disulfate S₂O₇²⁻.

From oxyanion to acid

Oxyanion names connect directly to acid names (see how to name acids):

Oxyanion	Acid	Acid name
perchlorate	HClO₄	perchloric acid
chlorate	HClO₃	chloric acid
chlorite	HClO₂	chlorous acid
hypochlorite	HClO	hypochlorous acid
nitrate	HNO₃	nitric acid
nitrite	HNO₂	nitrous acid
sulfate	H₂SO₄	sulfuric acid
sulfite	H₂SO₃	sulfurous acid

-ate → -ic acid; -ite → -ous acid. The prefixes per- and hypo- carry over unchanged.

Step-by-step: naming an unfamiliar oxyanion

Example: name BrO₂⁻.

1. Identify the element: bromine, a halogen — use the chlorine pattern.
2. Reference ion: bromate is BrO₃⁻ (like chlorate).
3. BrO₂⁻ has one fewer oxygen → bromite.

Example: write the formula of periodate.

1. Iodine, a halogen — like chlorine.
2. Iodate is IO₃⁻; per- adds one oxygen → IO₄⁻.

Example: name SeO₄²⁻.

1. Selenium is in group 16 below sulfur.
2. It follows the sulfate pattern: SO₄²⁻ is sulfate, so SeO₄²⁻ is selenate.

Where you’ll meet these ions

Oxyanions aren’t just exam material; they turn up throughout practical and everyday chemistry. Nitrates and phosphates are the main nutrients in fertilisers, and run-off of both into rivers and lakes can cause algal blooms. Sulfites are used as preservatives in wine and dried fruit because they’re reducing agents that stop oxidation. Hypochlorite is the active ingredient in household bleach and is used to disinfect drinking water and swimming pools. Chlorates and perchlorates are strong oxidisers used in fireworks, matches and rocket propellants, which is why they’re handled carefully. Carbonate and hydrogencarbonate control the pH of oceans and blood, and silicates make up most of the rocks in Earth’s crust.

Common mistakes

• Assuming -ate always means four oxygens. Nitrate and carbonate have three.
• Changing the charge between -ate and -ite. It stays the same.
• Confusing sulfide, sulfite and sulfate: S²⁻, SO₃²⁻, SO₄²⁻.
• Using hypo- with -ate or per- with -ite. It’s always per…ate and hypo…ite.
• Mixing up hydrogen ion names: HCO₃⁻ is hydrogencarbonate, not “hydrogen carbonate ion with charge 2−”.

Practice

Name: (a) NO₂⁻ (b) BrO₄⁻ (c) IO⁻ (d) HSO₃⁻ (e) SeO₃²⁻ Write formulas: (f) chlorite (g) hydrogenphosphate (h) perbromate

Answers: (a) nitrite (b) perbromate (c) hypoiodite (d) hydrogensulfite (e) selenite (f) ClO₂⁻ (g) HPO₄²⁻ (h) BrO₄⁻

Key takeaways

• -ate has one more oxygen than -ite; the charge is the same.
• per…ate has one more O than -ate; hypo…ite has one fewer O than -ite.
• Learn the -ate reference ion for each element; derive the others.
• Names track the oxidation state of the central atom.
• Hydrogen in front means an added H⁺ and one less negative charge; -ate → -ic acid, -ite → -ous acid.

For a full list of ions, see common ions and their charges.