Q. Magnetic Properties of f-block element?

You have learnt in the preceding unit-that paramagnetism is related with the presence of unpaired electrons in a substance. The actinide and lanthanide ions,  other than type (e.g.  La^3+, Ce4+, Ac³⁺, Th⁴⁺, Pa⁵⁺, U⁶⁺, and f¹⁴ type ( e, g Yb²⁺ Lu³⁺ Lr³⁺paramagnetic, due to each of the seven f orbitals charactering inner -transition metal  species (lanthanide and actinide) must have a single electron before any pairing can take  place (Hund's rule).

You have also studied that in case of transition elements; the contribution of orbital motion of electrons to paramagnetism is negligible and will be ignored. The magnetic moments of transition metal ions can be explained in terms of unpaired electrons present in d-orbitals.  But the magnetic moments of only those lanthanide ions, which have,f⁰ f⁷and f¹⁴ configuration agree with the spin only value. In all other cases, the magnetic moment value are higher than those evaluated on the basis of spin only formula. Though, these can be explained by taking orbital contribution to magnetic moment also into account. In lanthanide ions, the 4f orbitals are comparatively better shielded from the surroundings by the overlying 5p and 5s orbitals than the d orbitals in transition metal ions. Thus, the contribution of orbital motion to paramagnetism is not quenched. 

Although actinides show a variation in magnetic properties same to that of the lanthanides, the magnetic properties of the actinide ions are more complicated than those of the lanthanide ions. This in part arises from (i) the fact that the 5f electrons are nearer the surface of the atom and are easily influenced by the chemical environment; although not to the similar extent as do the d electrons, and (ii) the less sharply illustrate distinctions between 5f and 6d electrons as compared with 4f and 5d electrons. From the above discussion it is clear that the magnetic moments of the f-block (inner transition) metal ions have to be calculated taking into account both spin and orbital contributions.