Read the following passage and answer the next five questions based on it.
| Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
| Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd |
| La | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg |
In any transition series, as we move from left to right the d-orbitals are progressively filled and their properties vary accordingly.
| Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu |
| Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No | Lr |
The above are the two series of f-block elements in which the chemical properties won’t change much. The 5f-series elements are radioactive in nature and mostly are artificially synthesized in laboratories and thus much is not known about their chemical properties.
Why do the actinoids exhibit higher number of oxidation states than lanthanoids ?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Energy difference between 5f and 6d is less with respect to the energy difference between 4f and 5d.
An element shows many oxidation states when several of its outer electrons have similar energies, so that different numbers of them can be lost. In the actinoids, the 5f, 6d and 7s levels are of comparable energies, so a varying number of 5f electrons can take part in bonding along with the 6d and 7s electrons; uranium, for example, shows states up to +6 and neptunium up to +7.
In the lanthanoids, the 4f orbitals lie deep inside the atom and are much lower in energy than the 5d orbitals. The 4f electrons are seldom used, so the lanthanoids mainly show +3.
So actinoids show more oxidation states because the 5f–6d energy gap is smaller than the 4f–5d gap.
Why the other options are incorrect
Option (1) 4f orbitals are more diffused than the 5f orbitals. — Incorrect. It is the other way round; the 5f orbitals extend further out than the deep-seated 4f orbitals.
Option (3) Energy difference between 5f and 6d is more with respect to the energy difference between 4f and 5d. — Incorrect. A larger gap would make the 5f electrons harder to use and would give fewer oxidation states, not more.
Option (4) Actinoids are more reactive in nature than the lanthanoids. — Incorrect. Actinoids are indeed highly reactive metals, but reactivity does not decide the number of oxidation states; the energies of the outer orbitals do.
NCERT (2026-27 edition):
Oxidation States
