The Octet Rule

The Octet Rule

What is the Octet rule?

The propensity of atoms to have eight electrons in their valence shell is called octet rule. Atoms having less than eight electrons form more stable compounds. The electrons only from the sand p orbitals take part in the octet rule. The most likely elements which adhere to this rule are the first twenty elements due to their low atomic weight. 

Richard Abegg stated that the answer to the difference between the maximum positive and negative valence of elements is regularly eight. He formulated it as Abegg’s rule in the year 1904. Further, in the year 1916, it was used by Gilbert N. Lewis to formulate octet rule.

Octet rule exceptions

Some usual exceptions involve molecules that have an odd number of electrons, molecules with atoms comprising of more or less than eight electrons. The sand p block molecules have less number of odd electrons when compared to d and f block elements. In case there are more than eight electrons around an atom, then it is termed as an expanded valence molecule. Below we have mentioned three general exceptions to the octet rule:

  • Molecules with an odd number of electrons.
  • Example, molecules such as BCl3, where one or more than one atom has less than eight electrons.
  • Molecules with one or more than one atom comprising more than eight electrons, viz, SF6.

Octet rule examples

  • Ionic Example: Metallic and nonmetallic elements join together to form molecules. The bonding process happens in the following way – 
    • The nonmetals draw electrons from the metals
    • The nonmetals puls a negative electric charge 
    • This makes the metal positive and results in the electrostatic attraction of atoms. 
    • For instance, consider a chlorine atom. It has seven valence electrons. It experiences a sodium atom with one valence electron and withdraws its one valence electron. This leads to the filling of the outer shell. 

Limitations of the octet rule

There are three limitations of octet rule:

  • The valence shell consisting of an odd number of electrons with no formal charge.
  • The valence shell consisting of more than a specified number of electrons. For example in PCl5 extra electrons are accommodated by phosphorous in d orbitals.
  • The incomplete octet.

To learn various other chemistry concepts such as molecular orbital theory, exothermic reaction and more in an interesting way.

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