The regular decrease in the size of lanthanide ions from its first element to the last element is known as lanthanide contraction. It is due to greater effect of the increased nuclear charge than that of the screening effect. The same effect is exhibited by d block elements. Cause of the d-block contraction is the poor shielding of the nuclear charge by the electrons in the d orbitals. The outer valence electrons are more strongly attracted by the nucleus causing the observed increase in ionization potentials.