When the Octet Rule Fails
Sometimes it is just impossible to write a Lewis structure in which all of the atoms in a molecule obey the octet rule. This happens most often when an atom forms more than four bonds. Examples are PCl5 and SF6, in which there are five P-Cl bonds and six S-F bonds, respectively. Since each covalent bond requires the sharing of a pair of electrons, P and S must exceed eight electrons in their outer shells. The Lewis formula of these two molecules are shown below.
Elements in period 2 such as carbon or nitrogen, never exceed an octet simply because their valence shell, having n=2, can hold a maximum of only 8 electrons. Elements in periods below period 2, however, sometimes do exceed an octet, because their valence shells can hold more than 8 electrons. For example, the valence shell for elements in period 3, for which n=3, can hold a maximum of 18 electrons, and the valence shell for period 4 elements can hold as many as 32 electrons.
In some molecules (but not many), an atom has less than an octet. The most common examples are compounds of beryllium and boron.
                ..               ..         ..          ..         ..
·Be· + 2 ·Cl: -----> :Cl··Be··Cl: --> :Cl-Be-Cl:
                ..              ..          ..          ..         ..
Note: there is only 4e- around Be
                                    :Cl:                  ..
                                      ·                   :Cl:
 ·            ..                ..    ·     ..          ..  ..   ..
·B· + 3 ·Cl: ------> :Cl··B··Cl: --> :Cl-B-Cl:
             ..                ..        ..           ..       ..