The actinoid metals are all silvery in appearance but display a variety of structures. What is the reason for this structural variability?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Irregularities in their metallic radii, which are far greater than in the lanthanoids
The structure a metal takes depends on the size of its atoms. The metallic radii of the actinoids change irregularly along the series, far more than those of the lanthanoids, so different actinoid metals take up different structures.
Option (1) The radioactivity of the actinoids — Incorrect.
All actinoids are radioactive, and this makes them difficult to study (short half-lives of the later members, tiny amounts).
Radioactivity is a property of the nuclei; it does not decide how the atoms pack in the metal.
Option (2) The actinoid contraction, which is smaller than the lanthanoid contraction — Incorrect. The actinoid contraction is in fact greater from element to element than the lanthanoid contraction, because 5f electrons shield poorly. A contraction describes a gradual fall in size, not irregular changes.
Option (3) Irregularities in their metallic radii, which are far greater than in the lanthanoids — Correct. These irregular radii are the stated reason for the structural variability of the actinoid metals.
Option (4) The protective oxide layers formed on the metals — Incorrect. Protective oxide layers explain why most actinoids are only slightly affected by nitric acid; a surface layer does not decide the structure of the metal itself.
NCERT (2026-27 edition):
4.6.4 General Characteristics and Comparison with Lanthanoids – “The actinoid metals are all silvery in appearance but display a variety of structures. The structural variability is obtained due to irregularities in metallic radii which are far greater than in lanthanoids.”