Polyatomic Ions

Polyatomic Ions — Study Notes

1− ions

Formula

Name

Memory tip

NO₃⁻

Nitrate

-ate

NO₂⁻

Nitrite

-ite = one less O

SO₃²⁻

Sulfite

-ite

OH⁻

Hydroxide


NH₄⁺

Ammonium

Only positive ion on this sheet

C₂H₃O₂⁻ / CH₃COO⁻

Acetate

Two ways to write it

HCO₃⁻

Bicarbonate / Hydrogen carbonate


MnO₄⁻

Permanganate

per- = most O

ClO₃⁻

Chlorate


ClO₂⁻

Chlorite


ClO⁻

Hypochlorite

hypo- = least O

H₂PO₄⁻

Dihydrogen phosphate


CN⁻

Cyanide


ClO₄⁻

Perchlorate

per- = most O

2− ions

Formula

Name

SO₄²⁻

Sulfate

CO₃²⁻

Carbonate

CrO₄²⁻

Chromate

Cr₂O₇²⁻

Dichromate

O₂²⁻

Peroxide

S₂O₃²⁻

Thiosulfate

C₂O₄²⁻

Oxalate

3− ions

Formula

Name

PO₄³⁻

Phosphate

PO₃³⁻

Phosphite

⭐ Patterns to memorize

-ate vs. -ite

For many elements:

  • -ate = more oxygen

  • -ite = one fewer oxygen

Examples:

  • NO₃⁻ = nitrate

  • NO₂⁻ = nitrite

  • SO₄²⁻ = sulfate

  • SO₃²⁻ = sulfite

Chlorine series

Remember the oxygen count:

ClO⁻ → ClO₂⁻ → ClO₃⁻ → ClO₄⁻

hypochlorite → chlorite → chlorate → perchlorate

Think:

hypo- → -ite → -ate → per-
least O → → → most O

Phosphorus

  • PO₄³⁻ = phosphate

  • PO₃³⁻ = phosphite

  • HPO₄²⁻ = hydrogen phosphate

  • H₂PO₄⁻ = dihydrogen phosphate

⚠ Note: The sheet appears to print HPO₄⁻, but hydrogen phosphate is normally HPO₄²⁻. So this is likely a typo on the sheet.

Quick charge groups

  • 1−: most of the common ions here

  • 2−: sulfate, sulfite, carbonate, chromate, dichromate, peroxide, thiosulfate, oxalate

  • 3−: phosphate, phosphite

  • 1+: ammonium

Big ones to know cold:
NO₃⁻ nitrate, SO₄²⁻ sulfate, CO₃²⁻ carbonate, PO₄³⁻ phosphate, OH⁻ hydroxide, NH₄⁺ ammonium, C₂H₃O₂⁻ acetate.