2.3.1—Ionic formulas and oxidation numbers

An ionic formula represents a neutral ionic compound: the total positive charge from the cations must equal the total negative charge from the anions. Use the ion charges and choose the simplest whole-number ratio of ions that gives overall charge zero.

Use a fixed sequence: (1) write the cation and anion with their charges, (2) choose subscripts so total charge balances, (3) simplify the ratio if a common factor remains, and (4) remove charge labels from the final neutral formula. Do not change a polyatomic ion's internal subscripts.

If more than one polyatomic ion is needed, put the ion in brackets before adding its subscript, for example Ca²⁺ with NO₃⁻ gives Ca(NO₃)₂. Common ions such as NO₃⁻, CO₃²⁻, SO₄²⁻, OH⁻, NH₄⁺ and PO₄³⁻ must be treated as intact charged groups when constructing the formula.

Use a Roman numeral as the oxidation number of a variable-charge metal, not as a formula subscript: iron(III) is Fe³⁺ before balancing. Do not balance an equation by changing these formula subscripts; equation coefficients belong to the neighbouring objective.