Exceptions To
The Octet Rule
Odd Number of Electrons
When molecules have an odd number of
electrons, it is not possible to end up with an
octet around each atom
NO and ClO2
Less Than An Octet
Relatively rare situations, most often
involving compounds of B or Be
BF3 can be drawn with the following Lewis
structures
Being electron deficient allows the BF3
molecule to react very quickly with
electron-rich molecules
More Than An Octet
Largest class of octet exceptions
When the Lewis structure for PCl5 is drawn,
we are forced to “expand” the valence shell
and place 10 e− around the central P atom
Other examples with “expanded” valence
shells are SF4, AsF6−, and ICl4−
These exceptions are observed only for
central atoms from period 3 and beyond
Molecules such as NCl5 and OF4 do not
exist!
Elements of the second period have only the
2s and 2p valence orbitals available for
bonding
These orbitals can hold a maximum of 8
electrons (collectively)
Elements of the third period (and beyond)
have ns, np, and unfilled nd orbitals that can
be used for bonding
The larger the central atom is, the larger the
number of atoms that can surround it
Expanded valence shells occur most often
when the central atom is bonded to the
smallest and most electronegative atoms
(such as F, O, and Cl)
Homework:
Pg. 335 # 61, 62, 63, 64