Question

why is helium is having the smallest atomic radius

Answer 1

as helium has only 2 electrons and have only 1 shell....

and as compared to hydrogen the ratio atomic mass to the number of electrons is greater in helium...
as hydrogen has only one proton but helium have 2 protons and 2 neutrons

Answer 2

The helium atom is smaller than the hydrogen atom, because the nuclear charge is greater, and the 2 electrons occupy the same orbital as does the single electron in the hydrogen atom. This is a fundamental principle of inorganic chemistry.