Question

the atomic radius of helium is 31 and the atomic radius of the hydrogen is 53 then why hydrogen is said to be the smallest atom

Answer 1

There are different definitions of the atomic radius (van der Waals radius, covalent radius, metallic radius, etc.). For an isolated atom, the calculated value of the atomic radius for hydrogen is 53 pm, while that of helium is 31 pm. However, hydrogen exists as a diatomic while helium exists as a monoatomic. Therefore, comparing their calculated values doesn’t really make sense. The best way would be to compare van der Waals radii. The van der Waals radius of hydrogen is 120 pm while that of helium is 140 pm. (Data from authentic sources).

The answer to the argument that the atomic radius decreases across a period due to the increase in the number of protons in the nucleus is that the most genuine of reference books avoid including the atomic radii of noble gases in the periodic trend. These books generally use covalent radii while comparing periodic trend and thus the noble gases are excluded.