Define Lanthanide Contraction.

Explanation

The steady decrease in atomic and ionic size of Lanthanoid elements with increasing atomic number is called Lanthanoid contraction.
Cause : In the lanthanoid series, as we move from one element to another, the nuclear charge increases by one unit and one electron is added. The new electrons are added to the same inner 4f subshells. However the 4f electrons shield each other from the nuclear charge quite poorly because of the very diffused shapes of f-orbitals the nuclear charge. However increase by one at each step. Hence with increase atomic number the nuclear charge, the effective nuclear charge experience by each 4f-electron increases. As a result the whole of 4f-electron shell contracts at each successive element, though the decrease is very small. This result in gradual decrease in size of Lanthanoids with increase in atomic number. The sum of the successive reductions gives the total lanthanoid contraction.