Q.Assertion: Actinoids form relatively less stable complexes as compared to lanthanoids.
Reason: Actinoids can utilise their 5f orbitals along with 6d orbitals in bonding but lanthanoids do not use their 4f orbital for bonding.

Concept: Actinoid Contraction Greater – The key is that actinoids have more diffuse 5f orbitals which can participate in bonding, unlike the deeply buried 4f orbitals of lanthanoids.

Reasoning:

1. The assertion states actinoids form less stable complexes than lanthanoids. This is false — actinoids actually form more stable complexes due to greater covalent character. …

The key here is to separate two ideas that often get tangled: the extent of complex formation and the reason for it. Let’s start with the reason, because it’s the simpler part.

1. The reason is correct.

   In lanthanoids, the 4f orbitals are deeply buried inside the atom — they are shielded by the 5s and 5p orbitals and do not extend far enough to participate in bonding. Lanthanoid complexes are primarily ionic, with the metal using 6s and 5d orbitals (and sometimes 4f only in a very limited way).

   In actinoids, the 5f orbitals are more radially extended and less shielded. They can overlap with ligand orbitals, and the 6d orbitals are also available. This gives actinoids a greater ability to form covalent bonds.

2. Now the assertion: “Actinoids form relatively less stable complexes than lanthanoids.”

   This is the opposite of what actually happens. Because actinoids can use 5f and 6d orbitals for covalent bonding, they form more stable complexes than lanthanoids. The greater charge on actinoid ions (often +3, +4, or even +5) and the availability of f-orbitals for back-bonding also contribute to higher stability.

3. Why the confusion? …

Method: Cause-Effect Analysis for Assertion-Reason Questions

This method checks whether each statement is true/false and then tests if the reason correctly explains the assertion.

Steps

1. Verify the Assertion

   • Assertion: "Actinoids form relatively less stable complexes as compared to lanthanoids."
   • True. Actinoids have greater charge density and better orbital availability (5f, 6d, 7s) for covalent bonding, so they form more stable complexes — but the assertion says less stable.
   • Correction: Actinoids actually form more stable complexes than lanthanoids.
   • Hence, the assertion is false.

2. Verify the Reason

   • Reason: "Actinoids can utilise their 5f orbitals along with 6d orbitals in bonding but lanthanoids do not use their 4f orbital for bonding."
   • True. Lanthanoids have poorly shielding 4f orbitals that are buried inside the atom and do not participate in bonding. Actinoids have more extended 5f orbitals that can hybridise with 6d orbitals for covalent bonding. …

Here’s a breakdown of the common mistakes students make on this question and how to avoid each.

Mistake 1: Thinking the Assertion is True

Many students assume that because actinoids are radioactive and have complex chemistry, they form less stable complexes than lanthanoids. This is incorrect.

• Why it’s wrong: Actinoids actually form more stable complexes than lanthanoids. The reason is exactly what the Reason statement says — greater orbital participation (5f and 6d) leads to stronger bonding.
• How to avoid: Memorise the trend: Actinoid complexes > Lanthanoid complexes in stability. The greater availability of f-orbitals for bonding (due to lower energy and better spatial extension) makes actinoid complexes more stable, not less.

Mistake 2: Misinterpreting the Reason as False

Some students think the Reason is false because they believe lanthanoids do use 4f orbitals in bonding.

• Why it’s wrong: Lanthanoids generally do not use 4f orbitals for bonding — these orbitals are deeply buried and shielded. Actinoids, however, can use both 5f and 6d orbitals because 5f orbitals are more diffuse and less shielded.
• How to avoid: Remember the key difference:
  • Lanthanoids: 4f orbitals are core-like, poor in bonding.
  • Actinoids: 5f orbitals are more extended, can participate in bonding along with 6d.

Mistake 3: Choosing Option (i) — Both True, Reason Correct Explanation

This is the most common trap. Students see both statements as true and think the Reason explains the Assertion.

• Why it’s wrong: The Reason is true, but it explains the opposite of the Assertion. If actinoids use more orbitals for bonding, they should form more stable complexes — not less. So the Reason does not explain the (false) Assertion.
• How to avoid: Always check if the Reason logically supports the Assertion. Here, the Reason contradicts the Assertion. So even if both seem true individually, they don’t match.

Mistake 4: Choosing Option (ii) — Both True, Reason Not Correct Explanation

Some students realise the Reason doesn’t explain the Assertion, but they still think the Assertion is true.

• Why it’s wrong: The Assertion is false. Actinoids form more stable complexes, not less. So option (ii) is incorrect because it assumes the Assertion is true.
• How to avoid: Double-check the factual correctness of the Assertion first. If the Assertion is false, options (i) and (ii) are automatically eliminated.

Mistake 5: Choosing Option (iv) — Both False

A few students think the Reason is also false. …