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Define octet rule. Write its significance and limitations.

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According to “Electronic theory of chemical bonding” of chemical combination, atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells and become stable. This is known as “Octet Rule”. Significance of Octet Rule : Octet rule is helpful in understanding the bonding and structures of most of the organic compounds.

Limitations of Octet Rule :
1. In some compounds, the number of electrons surrounding the central atom is less than eight especially 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.

2. The elements of third period and beyond that have 3d orbitals in addition to 3s and 3p. In these elements, there are more than eight valence electrons around central metal atom. Some of the examples of such compounds are: PF6, SF6, H2SO4 and a number of coordination compounds.

3. In molecules with an odd number of electrons like nitric oxide and nitrogen dioxide, the octet rule is not satisfied for all the atoms. In NO, octet of nitrogen is not complete.

4. This theory does not account for shape of molecules.

5. It does not explain the relative stability of the molecules being totally silent about the energy of a molecule.

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