Covalent compounds aka Molecular compounds, are formed from non-metals that share electrons. Because there are many sharing between two non-metals, the formula cannot be guessed unless we have a naming system that reveals the atoms involved.

For this we use a set of prefixes:

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The only time we drop a prefix is if the mono is to appear at the beginning of the name.

E.g. CO = Carbon Monoxide

CO2 = Carbon Dioxide *Note we don't say 'mono carbon'.

This doesn't work for metals, only non-metals. There last element changes to an 'ide'.