Question

The pair that referred to as ‘chemical twins’ is:
(A) Ac, Cf
(B) Hf, Ta
(C) Tc, Re
(D) La, Ac

Answer 1

The physical and chemical properties of the given elements can be compared to know the similarity between them. Properties such as atomic size, their chemical properties, etc. can be taken into account to identify the chemical twins.

Complete Step-by step answer:
The elements of lanthanide series and actinide series of the periodic table exhibit a characteristic and special property in their atomic radius known as lanthanide and actinide contraction. Lanthanide contraction and actinide contraction is exhibited by lanthanides and actinides respectively. It is a trend in the size of these elements where the atomic radii of these two series of elements gradually decrease with the increasing atomic number throughout the series. The reason for the 4f and 5f electrons of the respective series is that they are not properly shielded, from the nuclear attractions by the electrons in s and p-orbitals.
This improper shielding makes the orbital attracted towards the nucleus and thus the size decreases gradually across the periods. Due to this decrease in size the elements exhibit similar chemical properties. The similarity in the properties is maximum in some pairs of elements belonging to these two series. Such pairs of elements with same properties are Zirconium (Zr) and Hafnium (Hf), Niobium (Nb) and tantalum (Ta), Molybdenum (Mo) and tungsten (W), Technetium (Tc) and Rhenium(Re) .
From the given options the pair of elements which are examples of chemical twins among the above mentioned pairs of elements are Technetium and rhenium. The atomic radius of Technetium is 205pm and that of rhenium is also 205pm.
Therefore the answer for the given question, the pair that is referred to as ‘chemical twins’ is option (C) Tc, Re.

Note: The similar nature of the elements is due to the lanthanide and actinide contractions. Technetium and rhenium belong to the same group that is group 7 but have the same atomic size and also elements of the same group have the same properties. Hence they have most resemblances and are considered as one of the chemical twin pairs.