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

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.

Answer 2

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