# What element has the smallest atomic radius?

Oct 29, 2016

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

#### Explanation:

And thus the element with the smallest atomic radius should be $\text{helium, Z=2}$.

This site reports that the atomic radius is $31 \times {10}^{-} 12 m$, where the atomic radius of the hydrogen atom is $53 \times {10}^{-} 12 m$.

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.