MHT CET Chemistry Question (2026) — Solution

Step-by-step solution

Lanthanide contraction refers to the steady decrease in the atomic and ionic radii of lanthanides with an increase in atomic number. As we move along the lanthanide series, the nuclear charge increases by one unit at each step, and the new electron enters the 4f subshell. The shape of the 4f orbitals is highly diffused, which results in a very poor shielding effect. Consequently, the shielding of one 4f electron by another in the same subshell is inadequate to counterbalance the increased nuclear charge. Due to this poor shielding, the effective nuclear charge experienced by the outermost electrons increases, pulling them closer to the nucleus and causing a contraction in size. Answer: Poor shielding of one of 4f electron by another in the sub-shell.