What element has the smallest atomic radius?

Hint:Atomic radius is the distance between the atom’s nucleus and the outermost shell or orbital of the electron. It varies greatly depending upon different types of forces and effects.

Complete step by step answer:
Atomic radii varies greatly depending upon a lot of factors.
Moving from to bottom vertically in a group the atomic radius increases since there is an increase in a number of shells or orbital. This means the count of shells increases by one as we go one step down and hence, the distance from the radius increases which is basically the distance between the electron and the nucleus. Due to this the attraction between them also decreases and hence it is easier to remove electrons as we go down.
Also, as we move left to right in a row horizontally the atomic radii decreases as the number of electrons increases due to this the attraction between the electrons and nucleus increases and this decreases the radius. Also since the attraction is increasing so it becomes difficult to remove electrons as we move left to right.
By this we can understand that as we move right and towards the top we will get the atom which has the smallest atomic radius.
After looking at the periodic table we get that HELIUM is the element in the eighteenth group and first row and hence our required answer with the smallest atomic radii.

Note: In the similar pattern we can go on finding elements of different atomic radius. We can see that neon will be our next element, the second smallest element. Also by the same pattern we can see that francium is the element with the largest atomic radius.