Question

chemical reactivity first decrease nd than increase in a period

Answer 1

yes because firstly number of valancy will increase and then decrease , on which the reactivity of an atom depends

Answer 2

the reactivity of element depend upon their tendency  to lose or gain electrons to complete their outermost orbit . greater the tendency to lose electron(s) , greater is the reactivity  in the case of metals . similarly greater  the tendency to gain electrons , greater is the reactivity of non metals .