On moving along the lanthanoid series, the atomic number increases. Also, with the increase in atomic number, the number of electrons in the 4f orbital also increases. The 4f electrons have a poor shielding effect. Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the force of attraction between the nucleus and valence electrons increases. This results in a decrease in the size of lanthanoids with the increase in the atomic number. This is termed as lanthanoid contraction.

CONSEQUENCES:

1. The properties of second and third transition series are similar in nature.

2. Separation of lanthanoids from its compounds is possible due to lanthanide contraction.

3. The variation in the basic strength of lanthanide hydroxides os due to this contraction. (Basic strength decreases from La(OH)₃ to Lu(OH)₃)