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Due to lanthanide contraction _____
(A) Fe, Co, Ni have equal size
(B) Zr and Hf have equal size
(C) All f-block ions have equal size
(D) All isoelectronic ions have equal size

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Last updated date: 13th Jun 2024
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Answer
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Hint: As we move down in the group, the size of element increases in the periodic table. Due to lanthanide contraction, the element present in the second and third transition series has relatable chemical and physical properties.

Complete answer:
The lanthanide contraction gives a description about the trend of the atomic radius in the lanthanide series.
The lanthanide contraction can be seen in all the 14 elements present in the lanthanide series. The elements present in this series are Cerium (Ce), Praseodymium(Pr), Neodymium(Nd), Promethium(Pm), Samarium(Sm), Europium (Eu), Gadolinium (GD), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb) and Lutetium (Lu).
According to the lanthanide contraction, the atomic radius of the element decreases with the increase in atomic number.
As we move down in the group, the atomic size increases from the first to second member of the series. Thus, the element presents in the second and third transition series possess similar characteristics because of lanthanide contraction. As a result there is a very minute difference in their size.
Thus element pairs present in the series which have almost similar properties are Zr-Hf, Mo-W, Nb-T etc.
Therefore, the correct option is B.

Note:
In lanthanide contraction the 5s and 5p orbitals get into the 4f orbital so that the 4f orbital is not shielded as the nuclear change increases which is the reason for the decrease in the atomic radius of the atom. This decrease in size takes place in the whole series.