(solved)Question 3.28 of NCERT Class XI Chemistry Chapter 3

$\displaystyle \underline{\underline{\text{Group I}}}$

The ionization enthalpy decreases from top to bottom because of the increasing size of the atom.

Hence, it is easier for the atom to lose its $\displaystyle ns^1$ electron to gain the stable noble gas configuration, and the reactivity increases!

$\displaystyle \underline{\underline{\text{Group 17}}}$

The electron gain enthalpy becomes less negative as we move down a group.

The electron gain enthalpy becomes more negative from bottom to top because of the decreasing size of the atom.

Hence, it is easier for the atom to add an np electron $\displaystyle ns^2np^5$ to gain the stable noble gas configuration, and the reactivity increases!

Note: F has less negative electron gain enthalpy than Cl but it turns out to be more reactive because of its highest electronegativity. Moreover, there are many intermediate steps in a chemical reaction. Electron gain enthalpy is not the only factor that decides the rate of a reaction!