Limitations of the Octet Rule
Limitations of the Octet Rule
The octet
rule, though useful, is not universal. It is quite useful for understanding the
structures of most of the organic compounds and it applies mainly to the second
period elements of the periodic table. There are three types of exceptions to
the octet rule.
The incomplete octet of
the central atom
In some
compounds, the number of electrons surrounding the central atom is less than
eight. This is especially the case with elements having less than four valence
electrons. Examples are LiCl, BeH2 and BCl3.
Li, Be and B
have 1,2 and 3 valence electrons only. Some other such compounds are AlCl3
and BF3.
Odd-electron
molecules
In molecules
with an odd number of electrons like nitric oxide, No and nitrogen dioxide, NO2,
the octet rule is not satisfied for all the atoms
The expanded octet
Elements in
and beyond the third period of the periodic table have, apart from 3s and 3p
orbitals, 3d orbitals also available for bonding. In a number of compounds of
these elements there are more than eight valence electrons around the central
atom. This is termed as the expanded octet. Obviously the octet rule does not
apply in such cases.
Some of the
examples of such compounds are: PF5, SF6, H2SO4,
and a number of coordination compounds.
Interestingly,
sulphur also forms many compounds in which the octet rule is obeyed. In sulphur
dichloride, the S atom has an octet of electrons around it.
Other drawbacks of
the octet theory
- It is clear
that octet rule is based upon the chemical inertness of noble gases. However,
some noble gases (for example xenon and krypton) also combine with oxygen and
fluorine to form a number of compounds like XeF2, KrF2,
XeOF2 etc.,
- This theory does not account for the shape of molecules.
- It does not explain the relative stability of the molecules being totally silent about the energy of a molecule.
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