[Wingman4l7]

I was about to correct you and say that hydrogen is the smallest, but apparently, hydrogen's atomic radius is actually a bit bigger. Helium's radius is smaller due to the larger charge of the nucleus making for a tighter electron cloud.

[manojlds]

I too had the same thought, and then thought maybe hydrogen molecule is being considered. Read up to see that hydrogen atom has bigger radius. TIL.

[jankotek]

Hydrogen creates molecules H2.

[Florin_Andrei]

Yup. Larger atomic radius due to less nuclear charge, and it's only stable in molecular form - H2.