The elements with the highest ionization energy, and thus the lowest reactivity, are:

noble gases

non-metals

Column VII

metals

alkali metals

Noble gases. You are welcome.

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.