what element has the smallest atomic radius? Li, K, F, or Cl. has the lowest ionization potential Li,O,S,or Cl

Elements get larger as you go down a group (column) in the periodic table because you are adding another shell to the atom each time. So Li < Na < K, etc. As you move across a period (row) the atomic size decreases because the electrons are being pulled in by the increasing number of protons in the nucleus. So Li > Be > B > C > N > O > F. Therefore, F is the smallest.

Ionization potential is the amount of energy required to remove an electron. For Group I metals like Li, this energy is very low since removing one electron gives Li+ an octet, which is very stable. Nonmetals, on the other hand, are looking to GAIN electrons to form an octet. Chlorine needs just one more electron for an octet, so it's going to be fairly difficult to REMOVE any electrons already there. The elements with the highest ionization energies are the noble gases; they HAVE an octet and certainly don't want to give up any electrons.

In general, ionization potential increases across a period (row) and decreases down a group (column). So O > S < Cl, but I wouldn't want to speculate between O and Cl since they are in different rows and columns.

Atomic Radius Of Li