Question

Explain in terms of subatomic particles, why the radius of a lithium ion is smaller than the radius of a lithium atom

Answer 1

lithium is a group one element that is alkali metals. All group one elements loses one electron to form ion.
The electronic configuration of lithium atom is 2.1 or 1s2 2s1
Lithium loses the one electron to form ion with electronic configuration of 2 or 1s2 for this reason the ionic radius of lithium is smaller than that of its atomic radius

Answer 2

An ion is a charged particle. In that sense, the particle may have one (or more) fewer electrons in the outer shells than the neutrally-charged lithium atom. In that case, the overall radius of the ion, having fewer subatomic particles to work with, will be slightly smaller than that of the atom.