Naming covalent compounds

Covalent compounds are named by counting the atoms of each element, using prefixes:

The rules:

• The first element keeps its name. Mono- is usually left out for it.
• The second element gets a prefix and its ending changes to -ide: oxygen becomes oxide, chlorine becomes chloride.

One spelling rule. If the prefix ends in 'o' or 'a' and comes before a vowel, drop that letter:

• CO: carbon monoxide (not mono-oxide);
• N₂O₄: dinitrogen tetroxide;
• N₂O₅: dinitrogen pentoxide.

An 'i' stays: carbon dioxide, sulfur trioxide.

Practise:

• CS₂: carbon disulfide.
• PCl₃: phosphorus trichloride.
• SF₆: sulfur hexafluoride.
• NO₂: nitrogen dioxide.

Two exceptions:

• No prefix before hydrogen when it comes first: H₂S is hydrogen sulfide, not dihydrogen sulfide.
• Some keep their common names: H₂O is water, NH₃ is ammonia.

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