All the alkali metals are highly reactive elements since they have a strong tendency to lose the single valence s-electron to form unipositive ions having inert gas configuration. This reactivity arises due to their low ionization enthalpies and high negative values of their standard electrode potentials.

However, the reactivity of halogens decreases with increase in atomic number due to following reasons.

As the size increases, the attraction for an additional electron by the nucleus becomes less.

Due to decrease in electronegativity from F to I, the bond between halogen and other elements becomes weaker and weaker.