Key ideas1 of 3
Charges determine the formula
A hydroxide ion has formula OH⁻. A metal ion with charge 2+ needs two hydroxide ions to produce a neutral formula, such as Cu(OH)2, Fe(OH)2, Ca(OH)2 or Mg(OH)2. Aluminium and iron(III) ions have charge 3+, so their hydroxides are Al(OH)3 and Fe(OH)3. Brackets show that the subscript applies to the whole OH group, not just hydrogen.
