3. "Unusual" Covalent Compounds.

So far all the compounds we have looked at have had complete outer shells of electron - stable noble gas configurations. There are however a few covalent compounds that break this rule - some have less than the usual eight electrons in the outer shell, and some have more!

An example of a compound with an incomplete outer shell is boron trifluoride - BF3.

Notice that although the fluorine atoms have full outer shells of electrons, the boron atom only has six electrons around it.
An example of a compound with an expanded outer shell is sulphur hexafluoride - SF6.

Notice that although the fluorine atoms have full outer shells of electrons, the sulphur atom has twelve electrons around it. The reasons why this can happen are not required at GCSE, but the sulphur atom has other energy levels that the extra electrons can go into.


Checkpoint 3.

Draw dot and cross diagrams for the molecules below. The first two have incomplete outer shells of electrons, and the last two have expanded outer shells.

a. BCl3
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b. BeCl2
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c. PF5
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d. BrF3
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