Why does ionisation result in a decrease in size for potassium but an increase in chlorine?
Ionization of potassium (K) results in a positive ion, K+, which
has the electronic configuration of Ar. The ionization of chlorine
(Cl) results in a negative ion (Cl-), also with the electronic
configuration of Ar. Adding an electron always increases the size
of an atom, and removing one always decreases its size.
Note that both K+ and Cl- have the same number of electrons, but
Cl- is larger than K+, because they are not as tightly held to the
nucleus in Cl- because it has 2 less protons than potassium does.
The size of the electron cloud determines the size of the atom and
with fewer positively charged protons to pull the same number of
electrons in close to the nucleus the electron cloud is larger for