Guides
How to Write Chemical Formulas — the Crossover Method
A chemical formula uses element symbols and subscripts to show what a substance is made of. The key to getting it right is making the positive and negative valences (charges) sum to zero, and reducing the ratio to the smallest whole numbers. With the crossover method, most inorganic compounds follow directly.
Basic rules
- The metal / positive element / ammonium goes first; the nonmetal / negative element / acid radical goes second.
- The whole formula is electrically neutral: total positive charge = total negative charge.
- When a polyatomic ion appears more than once, bracket it and put the subscript outside, e.g. (OH)₂, (SO₄)₃.
- Reduce the subscript ratio to smallest whole numbers.
The crossover method
Write each ion's charge magnitude as the other's subscript, then simplify:
- Al³⁺ with O²⁻ → Al₂O₃
- Al³⁺ with SO₄²⁻ → Al₂(SO₄)₃
- Ca²⁺ with Cl⁻ → CaCl₂
- Fe³⁺ with O²⁻ → Fe₂O₃; Fe²⁺ with O²⁻ → FeO
When the charges match, it is simply 1:1, e.g. Na⁺ with Cl⁻ → NaCl, Ca²⁺ with O²⁻ → CaO.
Common mistakes
- Forgetting to reduce: Mg²⁺ with O²⁻ is MgO, not Mg₂O₂.
- Missing brackets for repeated radicals: calcium nitrate is Ca(NO₃)₂, not CaNO₃₂.
- Writing the charge straight as a subscript without crossing over.
Look up charges in the valence table; once your formulas are right, build equations with the balancing guide.