* Atomic Radius Trend: Atomic radius generally increases as you move down a group (column) on the periodic table. This is because as you go down, you add more electron shells, which makes the atom larger.

* Lithium's Position: Lithium is at the top of Group 1 (alkali metals), while sodium is below it and hydrogen is even higher up (though it's not technically in the same group).

* Therefore: Lithium has the smallest atomic radius among these three elements.