Classification of Elements and Periodicity in Properties — Chemistry STD 11 Science — Question

The first ionization enthalpy generally increases as we go across a period and decreases as we descend in a group. As we go down a group, the outermost electron being increasingly farther from the nucleus, there is an increased shielding of the nuclear charge by the electrons in the inner levels. In this case, increase in shielding outweighs the increasing nuclear charge and the removal of the outermost electron requires less energy down the group.