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4.5     Define octet rule. Write its significance and limitations.

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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. This is known as the octet rule.

Significance: 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

LimitationsThere are three types of exceptions to the octet rule.

  • The incomplete octet of the central atom - the number of electrons surrounding the central atom is less than eight.Examples\ are\ LiCl, BeH_{2}\ and\ BCl_3.
  • Odd-electron molecules - the octet rule is not satisfied for all the atoms in  NO\ and\ NO_{2}.nono2
  • The expanded octet - there are more than eight valence electrons around the central atom. This is termed as the expanded octet. Some of the examples of such compounds are PF5, SF6, H2SO4 and a number of coordination compounds.pf5
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Divya Prakash Singh

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