Answer:
Step-by-step explanation:
Ionization energy is the amount of energy required to remove an electron from its valence shell.
Metals in general are electropositive i.e. they have a strong tendency to lose electrons and hence have the lowest ionization energy. Non-metals require an ionization energy which is greater than the metals. Column VII elements i.e. the halogens which have a valence electron configuration of ns²np⁵ are highly electronegative. Since they only require 1 electron to complete their octet they have a strong tendency to accept electrons.
In contrast, noble gases which have a valence electron configuration of ns²np⁶ already have a complete octet. This is a stable configuration which requires a lot of energy for electron removal. Hence noble gases constitute a group of elements with the highest ionization energy and lowest reactivity.