the electrons do not completely shield each other from the increasing nuclear charge as the number of protons in the nucleus increases. The electrons are expected to be bound more firmly in going from left to right across a given period, which means that the ionization energy should increase.the first ionization energy values decrease in going down a group.The main reason for the decrease in going down a group is that the electrons being removed are, on average, farther from the nucleus. As n increases, the size of the orbital increases, and the electron is easier to remove.