EXCEPTIONS TO THE OCTET RULE AND LEWIS STRUCTURES
As mentioned above, atoms found in or beyond the third period can have more than eight valence
electrons, since some of the valence electrons may occupy d orbitals. (Recall that in order to have
electrons in the d subshell, the principal quantum number must be 3 or above. The maximum
number of electrons that a shell can hold is 2n^2 ≥ 18 when n ≥ 3.) These atoms can be assigned more
than four bonds in Lewis structures. When drawing the Lewis structure of the sulfate ion, giving the
sulfur 12 valence electrons permits three of the five atoms to be assigned a formal charge of zero.
The sulfate ion can be drawn in six resonance forms, each with the two double bonds attached to a
different combination of oxygen atoms.
or two lone electron pairs can be taken from the N atom to form a triple bond between the C and N atoms.These three are all resonance structures of NCO–.