Question

Identify a ‘chemical twin’ among the following.
(A) Zr-Ta
(B) Nb-Tc
(C) Hf-Re
(D) Nb-Ta

Answer 1

The overall decrease in atomic and ionic radii in lanthanoids and actinoids was observed as lanthanoid and actinoid contraction. This will be explained based on the shielding effect. The electrons which are in inner shells, shielded the outermost electrons from nuclear charge make the experience low positive effective charge by the nucleus.

Complete step by step answer:
Due to the unique features observed in lanthanoids and actinoids which are 4f and 5f block elements, the size of atoms in this series will decrease with an increase in the atomic number. Because the impact of the shielding effect of one electron by another in the same subs shell is cause for this contraction.
A chemical twin is a pair of two elements having similar properties because of the same atomic radii. The pair of two elements which are in d-block elements of 4d and 5d series respectively. This is due to the lanthanide contraction.
In the given pair of elements, Nb-Ta has similar properties due to the same atomic radii.
So, the correct answer is “Option D”.

Note: In the lanthanide series, Zr-Hf is also called chemical twins in 4f block elements. Lanthanide contraction is responsible for these two elements that have the same atomic radii. Because 4f subshell in lanthanide poorly shields the outer electron from the nucleus causes a smaller size. So the 4f subshell of Hf has filled and not very effective the outer shell which is a similar size to Zr.