Question

Which element has the smallest radius?

Answer 1

Atomic radii decrease across a Period, but increase down a Group, so the element should be `He`.

Explanation:

So, given the above spray, we would predict that the smallest element is helium.

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` `"pm"`, and of helium as `0.31` `"pm"`; `1` `"pm"` `=` `1xx10^-12m`; inorganic chemists tend to use Angstroms, `1xx10^-10m`, but I can never find the Angstrom symbol, a capital "A" with a circle hat).