Why do atoms disobeying octet rule still stable ??

The octet rule applies to main group elements and to metals. But there are some stable except Helium has 2 electrons in its outer shell and is very stable, Hyrdogen only needs an additional one to achieve stability. In transition methods the octet rule does not apply and is instead replaced by the 18 electron rule .

Basically it can be seen as a result from the Pauli exclusion principle. For a given n-level the available angular momentum states, [imath] \ell [/imath], are given by [imath] n-1 \geq \ell \geq 0 [/imath]. So for lets say n=2, the available [imath] \ell [/imath] states are 1 and 0. These correspond to "s" and "p" orbitals. Thats 2+6...8 electrons. This holds for the elements in the p-block.

 

For [imath] n > 2 [/imath] there can be higher available [imath] \ell [/imath] levels but for elements in the p-block these levels will be lower in energy than the valence [imath] \ell = 1 [/imath] levels. So the valence shell will still tend to obey the octet rule.

Edited July 3, 201114 yr by mississippichem

There are also hypervalent compounds, which are stable sue to the nature of their bonding structure and the electronegativity of the associated ligands. If I have time later, I will expand on that some more.