9.5.1 Writing chemical formulae of covalent compounds

Last updated at July 3, 2026 by

💡 Valencies criss-crossed

For covalent compounds, criss-cross the valencies of the two elements and you have the formula.

Writing covalent formulae — worked steps
  • Hydrogen chloride: H (1) and Cl (1) → criss-cross → HCl (1s are not written).
  • Hydrogen sulfide: H (1) and S (2) → H₂S .
  • Carbon tetrachloride: C (4) and Cl (1) → CCl₄ .
Important Definitions
  • Covalent formula — written by criss-crossing the valencies of the two elements as subscripts.
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