Variation in first ionisation energies and atomic radiiFactors that affect ionisation energies: nuclear charge electron shielding distance from nucleus Ionisation energies generally increase across a period.Across a period: the number or protons increases, so there is more attraction acting on the electrons electrons are added to the same shell, so the outer shell is drawn inside slightly. There is the same number of inner shells, so electron shielding will hardly change. Across a period, the attraction between the nucleus and outer electrons increases, so more energy is needed to remove an electrons. This means that the first ionisation energy increases across a period.There is also a decrease in atomic radius across a period, because the increased nuclear change pulls the electrons in towards it.Starting the next period, there is a sharp decrease in ionisation energy between the end of one period and the start of the next.This is because there is another shell added, further from the nucleus so: increased distance of the outermost shell and the nucleus increased electron shielding of the outermost shell from the inner shells Trends Down a GroupDown a group ionisation energies decrease. Down each group: the number of shells increases, so the distance of electrons from the nucleus increases, hence there is a weaker force of attraction on the outer electrons there are more inner shells, so the shielding effect on the outer electrons from nuclear charge increases, hence less attraction The number of protons in the nucleus also increases, but the resulting increased attraction is far outweighed by the increase in distance and shielding. Taking all of these factors into account, the attraction between the nucleus and outer electrons decreases down a group, so less energy is needed to remove an electron. first ionisation energy decreases down a group atomic radius increases down a group, because less attraction means that the electrons are not pulled as close to the nucleus.