Question

The correct order of atomic radii is:
A.$$Ce>Sm>Yb>Lu$$
B. $$Sm>Ce>Lu>Yb$$
C. $$Lu>Yb>Sm>Ce$$
D. $$Sm>Yb>Ce>Lu$$

Answer 1

Hint: The given elements belong to a group of elements known as Lanthanides. Lanthanides are groups of elements which are present in the f- block and are also known as inner transition elements. The atomic number of this group of inner transition elements ranges from 58 to 71. These Lanthanides exhibit a special phenomenon known as Lanthanide contraction.

Complete step by step answer:
To solve this question, we must understand an important concept known as Lanthanide contraction.
Lanthanide contraction basically corresponds to the situation where the atomic radii of the elements in the Lanthanide series decreases drastically from left to right. Now we need to understand that f – block elements are placed in a designated zone inside the d – block. So once the last element of the lanthanide series, we do not move to the next period, but continue to place the d – block elements in the same period.
A general trend that is observed is that the atomic radius of elements decreases as we move from left to right in a period. But due to Lanthanide contraction, this trend is not observed. After the last element of lanthanide series, the atomic radius again increases when we move forward in the same period in the d – block.
Because of this phenomenon of lanthanide contraction, the correct order of atomic radii of the given elements can be given as: $$Sm>Ce>Lu>Yb$$ .

Hence, Option A is the correct option.

Note:
Lanthanide contraction is caused because of the poor shielding effect of f – orbital. This results in a greater nuclear change acting on the outer electrons and increases the force of attraction. This causes the outer electrons to be pulled strongly towards the nucleus, thus causing the rapid decrease in the radii of Lanthanides.