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The octet rule states that those substances (elements or compounds) are said to be stable if they have filled valence shells. Any substance reacts with other substances only to get stabilized by having a filled configuration i.e. duplet or octet. The reason for this inertness is the filled valence electronic shells of all the noble gases.

Noble gases belong to group 18 in the modern periodic table. They are known for their least reactive nature, hence named noble gases. Apart from this, they are also known as inert gases and zero group gases.

For example:

Electronic configuration for a few noble gases are:

$$\begin{gathered} He:1s^2 \\ Ne:1s^{2}2s^{2}2p^6 \\ Ar:1s^{2}2s^{2}2p^{6}3s^{2}3p^6 \end{gathered}$$

In the above configurations of the noble gases, it can be observed that for Helium with atomic number 2, the duplet is complete making it stable. Neon with atomic number 10 is also having all the shells filled and will not be participating in any bond formations. Similarly, Argon with atomic number 18 and all the higher noble gases also has the shells filled.