Why is the radius of the lithium atom considerably larger than that of the hydrogen atom?
The difference in the radii between hydrogen and lithium is due to the location of the valence electrons.
As you move down a group on the periodic table, successive energy levels are being added. The higher the energy level, the farther it is from the radius of the atom.
Hydrogen has one valence electron in the first energy level. The radius of lithium is larger than that of hydrogen, because the valence electrons of lithium are in the second energy level, which is farther from the nucleus than the first energy level occupied by the single hydrogen electron.