Assertion : Magnetic moment of Dy is the highest among langthanoids.
Reason : Orbital motion contributes magnetic moment.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reasonb is false
D. If both assertion and reason are false.

Explanation

To solve the assertion and reason question regarding the magnetic moment of Dysprosium (Dy) among lanthanides, we will analyze both the assertion and the reason step by step.

### Step 1: Understanding the Assertion
The assertion states that "the magnetic moment of Dy is the highest among lanthanides."

- Dysprosium (Dy) is a lanthanide element with atomic number 66.
- In lanthanides, the magnetic moment is primarily due to the presence of unpaired electrons in the f-orbitals, specifically the 4f orbital.

### Step 2: Evaluating the Magnetic Moment
The magnetic moment (μ) can be calculated using the formula:
\[ \mu = \sqrt{n(n + 2)} \]
where \( n \) is the number of unpaired electrons.

- Dysprosium has 5 unpaired electrons in its 4f orbital configuration.
- Therefore, substituting \( n = 5 \):
\[ \mu = \sqrt{5(5 + 2)} = \sqrt{35} \]

This indicates a relatively high magnetic moment compared to other lanthanides, confirming that Dy does indeed have one of the highest magnetic moments.

### Step 3: Understanding the Reason
The reason states that "orbital motion contributes to the magnetic moment."

- In lanthanides, the electrons in the 4f orbitals experience significant screening effects, which means that their orbital motion does contribute to the overall magnetic moment.
- The contribution from orbital motion is significant, especially when considering the unpaired electrons.

### Step 4: Conclusion
Both the assertion and the reason are correct. The assertion that Dy has the highest magnetic moment among lanthanides is supported by the reason that orbital motion contributes to the magnetic moment.

### Final Answer
Both the assertion and the reason are true, and the reason correctly explains the assertion.