The Prefix System

A third way of naming compounds where the first element may have more than one valence is the prefix system. This system is usually used for compounds containing two nonmetals, but the Stock System can still be used. A Greek or Latin prefix is attached to the name of the element.

 

The Prefix System

 Number

 Prefix

 1

mono-

2

di-

3

tri-

4

tetra-

 5

penta-

 6

hexa-

The prefix indicates the number of atoms of that element present in the compound. If only one atom of the first element is present in the formula, the prefix "mono-" is usually left out. For example, SO2 is written as sulphur dioxide and not monosulphur dioxide.

When a prefix ending with "o" or "a" is followed by "oxide", the "o" or "a" is omitted The compound N2O4 is named dinitrogen tetroxide and not dinitrogen tetraoxide.

And now a little practice!

1. NO2

2. PCl3

3. As2S3

4. silicon tetrachloride

5. nitrogen triiodide

6. dinitrogen trioxide

How about some practice with naming acids?

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