How would the reactivity of an element with one valence electron vary from an element with 8 valence electrons

An element with 1only one valence electron is extremely reactive while an element with 8 valence electrons is very stable and does not react most of the times

Explanation:

Valency is the number of electrons in the outermost shell of the atom. It shows the ease with which elements react and combine with each other.

An element can take a maximum of 8 electrons in most of its outermost shells. As a matter of fact, the goal for its stability is to attain a structure with all 8 slots for electrons in its outer shell filled up. The element can either do this by adding or removing electrons.

The ease with which it adds or removes the electrons shows its reactivity. If it has only one electron to lose, (the valence electron) the element will be very reactive because this is relatively easier to do when compared with 2 electrons, three electrons,  and so on. An element with 8 electrons in its outer shell is very stable and does not need to react anymore. an example of this is Neon and Argon.