Question

The lanthanide contraction is responsible for the fact that ___.
(A) Zr and Y have about the same radius
(B) Zr and Nb have similar oxidation state
(C) Zr and Hf have about the same radius
(D) Zr and Zn have similar oxidation state

Answer 1

Penetration describes the proximity to an electron approach to the nucleus. Electrons in different orbitals have different wave functions and also different radial distributions and probabilities. Lanthanide contraction is explained based on the shielding effect.

Complete step by step solution:
The f-block elements consist of two series lanthanides and actinides. The atoms of lanthanoids have an electronic configuration $6{{s}^{2}}$ with variable occupancy 4f level.
The overall decrease in atomic and ionic radii from lanthanum to lutetium is a unique feature in the chemistry of lanthanides. These phenomenons in lanthanides are known as lanthanide contraction. This will be explained based on the shielding effect core electrons in 6s orbital. In a multi-electron system, electrons are added to the outermost shells. The electrons which are in inner shells, shielded the outermost electrons from nuclear charge make them experience low positively effective charge by the nucleus.
The order of shielding electrons by inner electrons decreasing in the order.
s>p>d>f
This 4f subshell in lanthanide poorly shields the outer electron from the nucleus because of the smaller size. Hence, the 4f subshell of Hf has filled and is not very effective the outer shell (n=5 and n=6) which is a similar size to Zr.
So, the lanthanide contraction is responsible for the fact that Zr and Hf have about the same radius.

The correct answer is option C.

Note: Due to lanthanide contraction, the lanthanide series size of the atom decreases with an increase in the atomic number. Because the impact of shielding of one electron by another in the same subshell is cause for this contraction.