1. Helium has two electrons. Its electronic configuration is 1s2 . As per the configuration, it is supposed to be placed in ‘s’ block, but actually placed in s group which belongs to ‘p’ block. Because it has a completely filled valence shell as the other elements present in 18th group. It also resembles with 18th group elements in other properties.
Hence helium is placed with other noble gases.
2. The other exception is hydrogen. it has only one selectron 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 (17th 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.
