Arrange the following elements in decreasing order of their radii:
(A) La
(B) Pr
(C) Gd
(D) Yb
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B), (C), (D)
Across the lanthanoid series the atomic and ionic radii decrease steadily with increasing atomic number. This is the lanthanoid contraction. With each step one proton is added to the nucleus and one electron goes into a 4f orbital. The 4f electrons shield one another poorly from the nucleus, so the effective nuclear charge felt by the outer electrons rises at every step and the atom or ion becomes a little smaller.
Place the four elements by atomic number:
(A) La: Z = 57
(B) Pr: Z = 59
(C) Gd: Z = 64
(D) Yb: Z = 70
The radius falls as Z rises.
Shortcut: For a trend across a series, the order of radii is the order of atomic numbers. La is the lightest and Yb the heaviest, and only Option (2) starts with (A) and ends with (D).
NCERT (2026-27 edition):
Atomic and Ionic Sizes – “The overall decrease in atomic and ionic radii from lanthanum to lutetium (the lanthanoid contraction) is a unique feature in the chemistry of the lanthanoids.”