Question

The element with the smallest atomic radius is?

Answer 1

Helium

Explanation:

From electron filling of atomic shells we know that the smallest atomic radius will be of the element which has electrons having lowest principal quantum, `n=1`

As such we have only two elements which fill the `1s` orbital.
1. Hydrogen `1"s"1`
2. Helium `1"s"2`

We know that Hydrogen atom has one proton in its nucleus whereas helium atom has two protons. The double positive charge of the helium nucleus attracts its electrons more than single proton of hydrogen atom can attract its electron.
This Coulombs attractive force results in smaller radius of helium atom. This is confirmed by experiments where

Radius of hydrogen atom is found to be `53 " pm"`
Whereas Radius of helium atom is found to be `31 " pm"`