# Which element has the smallest radius?

Atomic radii decrease across a Period, but increase down a Group, so the element should be $H e$.
Nevertheless, given that we are physical scientists we should consider the data, and see if it confirms our prediction. This site lists the atomic radius of hydrogen as $0.37$ $\text{pm}$, and of helium as $0.31$ $\text{pm}$; $1$ $\text{pm}$ $=$ $1 \times {10}^{-} 12 m$; inorganic chemists tend to use Angstroms, $1 \times {10}^{-} 10 m$, but I can never find the Angstrom symbol, a capital "A" with a circle hat).