Assertion : 2s-orbital has one node.
Reason : Number of nodes in an orbital is equal to `(n - l -1)` value.

Options

A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statement but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement.

Answer

To solve the question, we need to analyze the assertion and reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the 2s orbital has one node.
- A node is a region in an atom where the probability of finding an electron is zero.

2. **Identifying the Type of Nodes**:
- There are two types of nodes: radial nodes and angular nodes.
- Radial nodes are calculated using the formula:
\[ \text{Radial Nodes} = n - l - 1 \]
- Angular nodes are equal to the azimuthal quantum number \( l \).

3. **Determining Quantum Numbers for 2s Orbital**:
- For the 2s orbital:
- The principal quantum number \( n \) is 2.
- The azimuthal quantum number \( l \) for s-orbitals is 0.

4. **Calculating the Number of Nodes**:
- **Radial Nodes Calculation**:
\[ \text{Radial Nodes} = n - l - 1 = 2 - 0 - 1 = 1 \]
- **Angular Nodes Calculation**:
\[ \text{Angular Nodes} = l = 0 \]
- Therefore, the total number of nodes in the 2s orbital is:
- Radial Nodes = 1
- Angular Nodes = 0
- Total Nodes = 1 (since only radial nodes contribute in this case).

5. **Conclusion**:
- The assertion is correct: the 2s orbital has one node.
- The reason provided is also correct: the number of nodes in an orbital is indeed calculated by the formula \( n - l - 1 \).
- Therefore, the reason correctly explains the assertion.

### Final Answer:
Both the assertion and the reason are correct, and the reason is the correct explanation for the assertion.