Question

How can we say that the reactivity of an element is high or the reactivity of an element is low????

Answer 1

The number of electrons in the outermost shell of an atom determines its reactivity. Noble gases have low reactivity because they have fully-filled electronic configuration. Hence they do not tend to react actively with other chemical species.

Halogens are highly reactive because they have 7 valence electrons in their outermost shell thus they readily gain an electron to fill their outermost shell