Question

Actinides exhibit a larger number of oxidation states than that of corresponding lanthanides. The reason behind this aspect is:
(a) lesser energy difference between 5f and 6d orbitals than between 4f and 5d orbitals.
(b) larger atomic size of actinides than the actinides.
(c) more energy difference between 5f and 6d orbitals than between 4f and 5d orbitals.
(d) greater reactive nature of the actinides than the lanthanides.

Answer 1

Hint: Both lanthanides and the actinides are called inner-transition elements or the f-block elements since in these elements the electrons are filled in the 4f and 5f orbitals respectively. These 4f and 5f orbitals are very poor shielders of the nuclear charge.

Complete answer:
The lanthanides and the actinides are present in between the third and the fourth group of the periodic table. The lanthanides belong to the sixth period while the actinides belong to the seventh period. Both of them have f-orbitals that are successively filled from Ce to Lu (in lanthanides) and from Th to Lr (in actinides).
 In lanthanides the 4f subshells are filled after the filling of the 6s subshell. Since the 4f subshell contains seven orbitals that can accommodate a total of fourteen electrons, there are a total of fourteen elements in the lanthanide series. They react similarly as alkaline earth metals.
 In actinides, the 5f subshell is filled after the filling of the 7s subshell. They are also a total of fourteen elements and all are radioactive.
 In actinides, both 4f and 5f orbitals are present. These orbitals are very poor shielders of the nuclear charge due to which the energy difference between the 5f and the 6d orbitals will be very low due to which the actinides will show a large number of oxidation states. This is not true for lanthanides since they have only the 4f orbitals.
So, the correct answer is “Option A”.

Note: As we move from Lanthanum to Lutetium we will see a gradual decrease in the atomic radii which is called the lanthanide contraction. It is caused due to the poor shielding f-orbitals due to which the effective nuclear charge increases as we move from Ce to Lu.