Why is helium placed in p-block elements although its last electron enters in the s-orbital ?

Explanation

He (Z = 2) with electronic configuration `1^(st)` is placed in group 18 (Noble gases) of p-block of elements. This is because He resembles other noble gases in its properties e.g. it is an inert gas like other noble gases. Further, due to fully filled outermost shell, like other noble gases, it has no tendency to lose gain or share electrons. Thus like Ne or Ar, it forms no compound.