Assertion (A) Actinoids form relatively less stable complexes as compared to lanthanoids.
Reason (R ) Actinoids can utilise their 5f orbitals alongwith 6d orbitals in bonding but lanthanoids do not use their 4f orbital for bonding.

Options

A. Both assertion and reason are true, and reason is the correct explanation of the assertion.
B. Both assertion and reason are true but reason is not the correct explanation of assertion.
C. Assertion is not true but reason is true.
D. Both assertion and reason are false.

Explanation

To analyze the given assertion and reason, we will break down the concepts involved and evaluate their validity step by step.

### Step 1: Understand the Assertion
The assertion states that "Actinoids form relatively less stable complexes as compared to lanthanoids."

- **Analysis**: Actinoids (elements 89 to 103) are known for their complex chemistry due to the involvement of their 5f orbitals. In contrast, lanthanoids (elements 57 to 71) primarily utilize their 4f orbitals. Generally, actinoids do form less stable complexes compared to lanthanoids due to their larger ionic sizes and higher oxidation states.

### Step 2: Understand the Reason
The reason states that "Actinoids can utilize their 5f orbitals along with 6d orbitals in bonding but lanthanoids do not use their 4f orbital for bonding."

- **Analysis**: Actinoids can indeed use their 5f and 6d orbitals for bonding, which allows for a greater variety of oxidation states and complex formation. On the other hand, lanthanoids primarily use their 4f orbitals, which are more shielded and less effective in bonding compared to the actinoids' orbitals. This statement is true.

### Step 3: Evaluate the Relationship Between Assertion and Reason
Now, we need to determine if the assertion is true or false and if the reason correctly explains the assertion.

- The assertion is **false** because actinoids do not form less stable complexes compared to lanthanoids; they are generally more unstable due to their larger ionic sizes and higher oxidation states.
- The reason is **true** because it accurately describes the bonding capabilities of actinoids and lanthanoids.

### Conclusion
Based on the analysis:
- The assertion is false.
- The reason is true.

Thus, the correct answer is that the assertion is not true, but the reason is true.

### Final Answer:
Assertion (A) is false, and Reason (R) is true.

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