9.5 Writing Chemical Formulae

Last updated at July 3, 2026 by

💡 The criss-cross trick

There is a quick shortcut for formulae: write the valencies, then criss-cross them as subscripts. Master it once and you can write almost any formula in seconds.

What is a chemical formula (the criss-cross idea)?
The Criss-cross Method Write the symbols Write the valencies or charges Criss-cross them as subscripts A valency of 1 is not written
  • Write the symbols of the elements/ions.
  • Write their valencies (or charges).
  • Criss-cross the valencies as subscripts.
  • A valency of 1 after criss-crossing is not written .
Important Definitions
  • Chemical formula — a shorthand showing the elements and the number of atoms or ratio of ions in a compound.
  • Valency — the combining capacity of an element, used in the criss-cross method.
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