The steady decrease in atomic and ionic lanthanoid elements with increasing atomic number is called lanthanoid contraction.
- As we move along the lanthanoid series, the atomic number gradually increases by one, or we can say the number of electrons and protons increases by one.
- The addition of electrons to the same shell increases the effective nuclear charge.
- An increase in atomic number also increases the number of electrons in the 4f orbital with a poor shielding effect. The effective nuclear charge upon the outer electrons also increases.
- Therefore, the size of lanthanoids steadily decreases with the increase in the atomic number, and the phenomenon is called lanthanoid.
- As a result of lanthanoid contraction, there is a similarity in the second and third transition series properties.