If you belive Wikipedia https://en.wikipedia.org/wiki/Atomic_radius https://en.wikipedia.org/wiki/Atomic_radius then the atomic radius of Helium has never been

If you belive Wikipedia https://en.wikipedia.org/wiki/Atomic_radius https://en.wikipedia.org/wiki/Atomic_radius then the atomic radius of Helium has never been measured, but it is predicted to be smaller than Hydrogen.