Limitations of octet rule :
i. Octet rule does not explain stability of some molecules.
The octet rule is based on the inert behaviour of noble gases, which have their octet complete i.e., have eight electrons in their valence shell. It is very useful to explain the structures and stability of organic molecules.
However, there are many molecules whose existence cannot be explained by the octet theory.
The central atoms in these molecules does not have eight electrons in their valence shell, and yet they are stable.
Such molecules can be categorized as having :
a. Incomplete octet
b. Expanded octet
c. Odd electrons
a. Molecules with incomplete octet :
e.g. BF3, BeCl2, LiCl
In these covalent molecules,
The atoms B, Be and Li have less than eight electrons in their valence shell but these molecules are stable. Li in LiCl has only two electrons, Be in BeCl2 has four electrons while B in BF3 has six electrons in the valence shell.
b. Molecules with expanded octet :
Some molecules like SF6, PCl5, H2SO4 have more than eight electrons around the central atom.
c. Odd electron molecules :
Some molecules like NO (nitric oxide) and NO2 (nitrogen dioxide) do not obey the octet rule.
These molecules, have odd number of valence electrons.
ii. The observed shape and geometry of a molecule, cannot be explained, by the octet rule.
iii. Octet rule fails to explain the difference in energies of molecules, though all the covalent bonds are formed in an identical manner, that is, by sharing a pair of electrons. The rule fails to explain the differences in reactivities of different molecules.
Note :
Sulphur also forms many compounds in which octet rule is obeyed. For example, in sulphur dichloride, the sulphur atom has eight electrons around it.
