Question

how to determine reactivity of an element using periodic table?

Answer 1

by seeing whether it is down the group or above and whther it is left side or right side in the period 

Answer 2

The number of electrons in the outermost shell of an atom determines its reactivity. Noble gases have low reactivity because they have full electron shells. Halogens are highly reactive because they readily gain an electron to fill their outermost shell.

Hope it helps...