What's a molecule that is an exception to the octet rule, and why is it considered an exception?

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ncchemist | eNotes Employee

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One chemical that is an exception to the octet rule is BCl3, or boron trichloride.  Boron is a group 13 element, meaning it has 3 valence electrons.  So each of these three valence electrons forms a single bond with a chlorine atom.  This gives a total of 3*2=6 electrons surrounding boron, which is two less than the standard octet.  As a result, BCl3 is a highly reactive compound and it acts as a Lewis acid, meaning that it readily accepts a pair of electrons from an electron donor to make an octet.