The configuration of lanthanides shows that the additional electron enters the 4f subshell. The shielding of one 4f electron by another is very little or imperfect. The imperfect shielding of f electrons is due to the shape of f orbitals which is very much diffused. Thus as the atomic number increases, the nuclear charge increases by unity at each step, while no comparable increase in the mutual shielding effect of 4 foccurs. This causes a contraction in the size of the 4f subshell as a result of which atomic and ionic radii decrease gradually from La to Lu.