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How do we write a covalent formula?
Step 1 — Write the symbols
Place the two elements side by side.
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Step 2 — Write each valency
Put each element's valency above its own symbol.
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Step 3 — Criss-cross them
Each valency becomes the other element's subscript.
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The formula — a subscript of 1 is not written. H (1) and Cl (1) give
HCl
; H (1) and S (2) give
H₂S
.
💡 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.
