NEET2026ChemistryChapterActual
While moving along the inner transition elements, the element-to-element decrease in the ionic radii of M³⁺ ions is observed to be more significant in the actinoid series compared to the lanthanoid series. Which of the following statements correctly explains this observation?
Options
- AThe 5f electrons shield the increasing nuclear charge less effectively than the 4f electrons.
- BThe 5f orbitals are more deeply buried in the atomic core than the 4f orbitals.
- CThe 4f electrons exhibit a weaker penetration effect towards the nucleus than the 5f electrons.
- DThe energy gap between 5f and 6d orbitals is much larger than that between 4f and 5d orbitals.
Correct answer
A. The 5f electrons shield the increasing nuclear charge less effectively than the 4f electrons.
Step-by-step solution
The decrease in ionic radii across the actinoid series (actinoid contraction) is more significant from element to element than the corresponding decrease in the lanthanoid series (lanthanoid contraction). This occurs because the 5f orbitals are more diffuse and extend further from the nucleus compared to the 4f orbitals. As a result, the 5f electrons shield the increasing nuclear charge less effectively than the 4f electrons. Due to this poorer shielding by 5f electrons, the effective nuclear charge experienced by