Question

What element has the smallest atomic radius?

Answer 1

Atomic size decreases across a Period, but INCREASES down a Group.

Explanation:

And thus the element with the smallest atomic radius should be `"helium, Z=2"`.

This site reports that the atomic radius is `31xx10^-12m`, where the atomic radius of the hydrogen atom is `53xx10^-12m`.

That atomic size should decrease across a Period from left to right as we face the Table, can be attributed to the increase of nuclear charge (i.e. the increase in `Z`), versus the inability of incomplete electronic shells to shield the nuclear charge effectively. The helium atom is smaller than the hydrogen atom, because the nuclear charge is greater, and the 2 electrons occupy the same orbital as does the single electron in the hydrogen atom.

This is a fundamental principle of inorganic chemistry.