The chemical reactivity is defined as the tendency of a compound to undergo chemical changes.

The elements with octet configuration are the most stable.

Along a period, the alkali metals and halogens are the most reactive. This is due to the fact that alkali metals can easily lose one electron to form a cation with noble gas configuration.

Similarly, the halogens need only one electron to obtain stability.

Thus, as we move along a period the tendency to lose electron decreases for the metals and tendency to gain electron increases for the non-metals. Consequently, the reactivity first decreases then increases.

As we go down a group in metals, the atomic size goes on increasing, nuclear charge decreases, thus the tendency to lose electrons increases and consequently reactivity increases.

As we move down a group in halogens, the size increase, nuclear charge decreases thus the tendency to gain electron decreases and consequently the reactivity decreases.

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 .

The chemical reactivity is defined as the tendency of a compound to undergo chemical changes.

The elements with octet configuration are the most stable.

Along a period, the alkali metals and halogens are the most reactive. This is due to the fact that alkali metals can easily lose one electron to form a cation with noble gas configuration.

Similarly, the halogens need only one electron to obtain stability.

Thus, as we move along a period the tendency to lose electron decreases for the metals and tendency to gain electron increases for the non-metals. Consequently, the reactivity first decreases then increases.

As we go down a group in metals, the atomic size goes on increasing, nuclear charge decreases, thus the tendency to lose electrons increases and consequently reactivity increases.

As we move down a group in halogens, the size increase, nuclear charge decreases thus the tendency to gain electron decreases and consequently the reactivity decreases.