AS Level Chem - Ions Need to Know

Formula for Ammonium?

[NH₄]⁺

Formula for Zinc?

Zn²⁺

Formula for Silver?

Ag⁺

Formula for Carbonate?

[CO₃]²⁻

Formula for Sulfate?

[SO₄]²⁻

Formula for Phosphate?

[PO₄]³⁻

Formula for Hydrogen Carbonate?

[HCO₃]⁻

Formula for Nitrate

[NO₃]⁻

Formula for Hydroxide?

[OH]⁻

Formula for Iron (III) Oxide?

Fe₂O₃ 

The Roman Numerals after the iron —  which is a transition metal — indicates its charge.