Explain the difference in the first ionisation energies of chlorine and bromine.

yes that is very close configuration between each other

As Br is smaller than Cl; therefore, Br- have a smaller ionization energy than Cl-. In short, since the outermost electron in bromine is farther from the nucleus than the outermost electron in chloride, it takes less energy to remove the outermost electron in bromide.

a