Octet rule deals with the stability of an atom. According to this rule, an atom is stable if its outermost shell have total of 8 e–. It is a special case of the stability rule of atoms which suggests that the outermost shell should be completely filled for an atom to be stable. The other case of stability rule is duplet rule. Although, the octet rule is not universal, it is useful in understanding the structures of most of the organic compounds. The octet rule mainly applies to the second and third period elements of the modern periodic table.
There are mainly three limitations of the octet rule:
The incomplete octet of the central atom
Any bond is not completely ionic or completely covalent, when the octet rule is applied to some molecules which have almost equal ionic and covalent characteristics, this rule fails.
For example: In some polyatomic molecules like LiCl, BeH2 and BCl3, the number of electrons surrounding the central atom is less than eight. Li, Be and B have 1,2 and 3 valence electrons only. This is especially the case with elements having less than four electrons in their valence shell. Examples of other such compounds are AlCl3 and BF3.
Molecules having an odd number of electrons like nitric oxide, NO and nitrogen dioxide, NO2, do not satisfy the octet rule for all the atoms.
The expanded atom
Elements in the third period of the periodic table and beyond have 3d orbitals, (apart from 3s and 3p orbital) available for bonding. As we know that the atom will be stable when the outermost shell is completely filled. In this case (4th period) the completely filled outermost shell can accommodate 18 e–. Numerous compounds of these elements have more than eight valence electrons around the central atom. This is termed as the expanded octet. Some examples of such compounds are: PF5, SF6, H2SO4 and a number of coordination compounds.
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