Ionization Energy in Groups:
Ionization energy values decreases from top to
bottom in a group. The reason is that extra shell are added in the groups. Due
to addition of shell distance between nucleus and outer most shell increases
and force of attraction decreases. Therefore ionization energy decreases from
top to bottom in a group.
Ionization Energy in Periods:
Ionization energy increases from left to right
in a period. The reason is that , there is no addition of extra shell in
periods. So attraction between nucleus and valance shell electron increases.
Therefor ionization energy increases in period.