Question

what conclusion can be drawn about the reactivity of an atom from its valency

Answer 1

An atom will always try to acquire an octet or acquire a state of stability by giving,taking or sharing electrons.The valency of an atom determines its combining capacity or the no. of electron it will gain,lose or share to become stable.
For eg= chlorine's electronic configuration=2,8,7
And it will gain 1 electron to become stable hence,its valency=1

Answer 2

Valency is the number of electrons in outermost shell of an atom. the number of electrons gained or lost or shared gives as the combining capacity of an atom and this decides whether an atom is reactive or not.