(a) : As we proceed from one element to the next element in the lanthanide series, the nuclear charge, i.e. atomic number increases by one unit and the addition of one electron occurs at the same time in 4f-energy shell. On account of the very diffused shapes of f-orbitals, the 4f-electrons shield each other quite poorly from the nuclear charge. Thus, the effect of nuclear charge increase is somewhat more than the changed shielding effect. This brings the valence shell nearer to the nucleus and hence the size of atom or ion goes on decreasing as we move in the series. The sum of the successive reactions is equal to the total lanthanide contraction.
