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Class 12 › Chemistry › The d- and f-Block Elements
CBSE2026Class Class 12 · Chemistry1 Marks · Long✅ Verified
Assertion (A) : Actinoids show wide range of oxidation states.
Reason (R) : Actinoids are radioactive in nature.
Codes: (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
✅ Answer & Solution
Step 1 - Examine the Assertion (A).
In actinoids the $5f$, $6d$ and $7s$ orbitals lie very close in energy (comparable
energies), so electrons from all three can take part in bonding. As a result actinoids
show a WIDE RANGE of oxidation states, from $+3$ up to $+7$.
For example, uranium shows $+3, +4, +5$ and $+6$; neptunium shows up to $+7$.
Assertion (A) is TRUE.
Step 2 - Examine the Reason (R).
Every actinoid element is RADIOACTIVE - all of them have unstable nuclei that undergo
spontaneous decay.
Reason (R) is TRUE.
Step 3 - Test whether (R) explains (A).
Radioactivity is a NUCLEAR property; it depends on the stability of the nucleus.
Oxidation states are a property of the EXTRA-NUCLEAR electrons, determined by the
closeness of the $5f$, $6d$ and $7s$ orbital energies. The two are completely
unrelated, so (R) does NOT explain (A).
Answer: (B) - Both (A) and (R) are true, but (R) is not the correct explanation of (A).
