A) inert-pair effect
B) screening effect
C) lanthanide contraction
D) increasing effective nuclear charge
Correct Answer: C
Solution :
In general, the size of the atoms increases down a group but the size of any atom of the third transition series (after lanthanum) is nearly same as that of the atom of the element lying in the same group of the second transition series. This is because, in the transition series after La, there are 14 lanthanoid elements and in lanthanoid series, there is a regular decrease in the atomic and ionic radii with increasing atomic number, which is known as lanthanoid contraction. Thus, the similarity in the size of atoms of the elements belonging to the same group of the 2nd and 3rd transition series is due to the effect of lanthanoid contraction.
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