What element has the smallest atomic radius?

1 Answer
Oct 29, 2016


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


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.