Assertion (A): Sodium forms a monovalent ion.
Reason (R): Sodium loses only one electron from the second last shell.

Options

A. Both A and R are true and R is the correct explanation for A.
B. Both A and R are true but R is not the correct explanation for A.
C. A is true and R is false.
D. A is false and R is true.

Explanation

To solve the question regarding the assertion and reason about sodium, we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Sodium forms a monovalent ion."
- A monovalent ion is one that has a charge of +1 or -1. In the case of sodium (Na), it forms a positive ion (Na⁺) by losing one electron.

2. **Understanding the Reason (R)**:
- The reason states that "Sodium loses only one electron from the second last shell."
- To evaluate this, we need to look at the electronic configuration of sodium.

3. **Electronic Configuration of Sodium**:
- Sodium has an atomic number of 11, which means it has 11 electrons.
- The electronic configuration can be written as follows:
- K shell (1st shell): 2 electrons
- L shell (2nd shell): 8 electrons
- M shell (3rd shell): 1 electron
- Therefore, the electronic configuration of sodium is 2, 8, 1.

4. **Identifying the Valence Shell**:
- The valence shell is the outermost shell that contains electrons. For sodium, the M shell is the valence shell, which contains 1 electron.
- When sodium loses this single electron from the M shell, it achieves a stable electronic configuration of 2, 8 (the configuration of neon, a noble gas).

5. **Conclusion on the Assertion and Reason**:
- The assertion (A) is true because sodium does indeed form a monovalent ion (Na⁺).
- The reason (R) is false because sodium loses the electron from its valence shell (M shell), not from the second last shell (L shell).

6. **Final Answer**:
- Since the assertion is true and the reason is false, the correct conclusion is that only A is true, and R is false.