The octet rule is insufficient for
describing the covalent bonding of molecules.
Using only the octet rule, the chemist might draw an oxygen
or a nitrogen with four bonds around it. This would be
incorrect.
Earlier it was said that in some cases, learning one rule
wasn't enough--that a set of rules had to be learned to
perform an operation. This is one of those instances.
Carbon prefers to have four bonds, and no lone pair of electrons.
Oxygen prefers to have two bonds and two lone pairs of
electrons.
Nitrogen prefers to form three bonds and have one lone pair
of electrons.
Formal Charge = # of valence electronz - (# of bonds + # of lone pair electrons)
C, 4 - (4 + 0) = 4 - 4 = 0
N, 5 - (3 + 2) = 5 - 5 = 0
O, 6 - (2 + 4) = 6 - 6 = 0
You will apply the concept of formal charge in Organic Chemistry
when you begin drawing reaction mechanism. To see an
animated reaction, try the
Transesterificatoin in Polyester Synthesis page at the USM Polymer Science Department's
Macrogalleria.
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