What are the two exceptions of block division in the periodic table?

Explanation

(i) Helium has two electrons. Its electronic configuration is `1s^(2)`?. As per the configuration, is supposed to be placed in .s. block, but actually placed in `18^(th)` group which belongs to ‘p’ block. Because it has a completely filled valence shell as the other elements presenti `18^(th)` group. It also resembles with `18^(th)` group elements in other properties. Hence helium placed with other noble gases.
(ii) The other exception is hydrogen. It has only one s-electron and hence can be placed in group 1. It can also gain an electron to achieve a noble gas arrangement and hence it can behave as halogens (`17^(th)` group elements). Because of these assumptions, position of hydrogen becomes a special case. Finally, it is placed separately at the top of the periodic table.