Assertion (A): Actinoids show wide range of oxidation states.
Reason (R): Actinoids are radioactive in nature.
Assertion (A): Actinoids show wide range of oxidation states.
Reason (R): Actinoids are radioactive in nature.
Options
Correct option: (B) — Both A and R are true, but R is not the correct explanation of A.
Why A is true:
- Actinoids exhibit a wide range of oxidation states (e.g. +3, +4, +5, +6, +7) because the energies of the 5f, 6d and 7s orbitals are very close, allowing many electrons to take part in bonding.
Why R is true but not the explanation:
- Actinoids are indeed all radioactive.
- However, radioactivity is a nuclear property, unrelated to the electronic (orbital-energy) reason behind the variable oxidation states.
- So both statements are true, but R does not explain A.
Marking Scheme
- 11 mark: correct code (B) — both statements true but Reason is not the correct explanation of the Assertion.
- 2Full mark for (B); no partial credit.
Hint
Both statements are correct facts — but ask whether radioactivity (a nuclear property) can explain variable oxidation states (an orbital-energy property).
Quick Oral Answer
Both statements are true, but radioactivity is a nuclear property while the wide range of oxidation states comes from the comparable energies of the 5f, 6d and 7s orbitals, so the reason does not explain the assertion — code (B).
Analysis & Explanation
Concept:
The variable oxidation states of actinoids come from the close, comparable energies of the 5f, 6d and 7s subshells. Because these levels lie near one another, a variable number of electrons can be lost, giving states from +3 up to +7 (e.g. uranium shows +3, +4, +5 and +6).
Contrast with lanthanoids:
- In lanthanoids the 4f electrons are buried deep and poorly shielded, so +3 dominates.
- In actinoids the 5f electrons are less strongly held and more available for bonding, giving a wider range of states.
Why the reason fails:
- Radioactivity results from unstable nuclei (nuclear property).
- The oxidation-state range is an electronic/chemical property.
- Two independent true facts — hence code (B), not (A).
Real-world:
Uranium's multiple oxidation states are exploited in the nuclear fuel cycle, where U(IV) and U(VI) species (e.g. UO₂²⁺) are interconverted during enrichment and reprocessing.
Common Mistakes
- 1Choosing (A) because both statements are true, without checking that R actually explains A.
- 2Believing radioactivity causes the range of oxidation states — it is the near-equal 5f/6d/7s orbital energies that do so.
- 3Confusing actinoid behaviour with lanthanoids, where +3 is the dominant, near-exclusive oxidation state.
Interesting Facts
Uranium alone shows oxidation states +3, +4, +5 and +6, a versatility central to nuclear fuel chemistry.
All elements beyond uranium (the transuranium actinoids like neptunium and plutonium) are synthetic and were first made during the 1940s Manhattan Project era.
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Frequently Asked Questions
Why do actinoids show a wide range of oxidation states?
Because the 5f, 6d and 7s orbitals have very similar energies, a variable number of electrons can participate in bonding. This allows oxidation states ranging from +3 up to +7, unlike lanthanoids where +3 dominates.
Does radioactivity cause the variable oxidation states of actinoids?
No. Radioactivity is a nuclear property arising from unstable nuclei, whereas the variable oxidation states are an electronic property governed by the comparable energies of the 5f, 6d and 7s orbitals. The two are unrelated.