Question

Following statements regarding the periodic trends of chemical reactivity of alkali metals and halogens are given. Which of these statements gives the correct picture?

• A. Chemical reactivity increases with increase in atomic number down the group in both the alkali metals and halogens
• B. In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group
• C. The reactivity decreases in the alkali metals but increases in the halogens with increase in atomic number down the group
• D. In case of both, alkali metals and halogens, chemical reactivity decreases with increase in atomic number down the group

Answer 1

The correct option is B In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group

The alkali metals are highly reactive because their first ionisation potential is very low and hence they have great tendency to give up electron to form unipositive ion.
On moving down in group $I$, from $Li$ to $Cs$, ionisation enthalpy decreases. Hence, the reactivity increases.

The halogens are highly reactive elements due to their high electron affinity i.e., their high tendency to accept an electron.
On moving down the group, the electron affinity decreases which decreases their oxidizing power. So, the reactivity also decreases down the group.