Periodicity

Periodicity is the repeating trend in the properties of the elements across a periodThese include electron configuration, ionisation energy, structure and melting points. Down a group the nuclear charge increases and there is an increase in shielding and an increased distance from nucleus due to the additional shells.As we go across a period, the nuclear charge increases but the electrons are in the same quantum shell, so there is similar shielding.Shielding refers to the repulsion of the outer electrons by the filled inner shell electrons. This reduces the attraction of the nucleus for the outer electrons.