Assertion :- Cr belong to 4th period
Reason :- Cr atom contain 4 shells.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True, but the Reason is False.
D. If both Assertion & Reason are false

Answer

To solve the question regarding the assertion and reason about Chromium (Cr), we will analyze both statements step by step.

### Step 1: Analyze the Assertion
The assertion states that "Cr belongs to the 4th period."

- **Explanation**: The periodic table is organized into periods (horizontal rows) and groups (vertical columns). The period number corresponds to the highest principal quantum number (n) of the electrons in the atom. Chromium has an atomic number of 24, and its electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹. The highest principal quantum number here is 4, indicating that Chromium is indeed in the 4th period.

### Step 2: Analyze the Reason
The reason states that "Cr atom contains 4 shells."

- **Explanation**: The term "shell" refers to the principal energy levels (n=1, n=2, n=3, n=4, etc.) that electrons occupy. For Chromium, the electronic configuration shows that there are electrons in the 1st, 2nd, 3rd, and 4th shells. Therefore, Chromium does contain 4 shells.

### Step 3: Conclusion
Both the assertion and the reason are correct. Furthermore, the reason provided is a correct explanation of the assertion because the presence of 4 shells (energy levels) in Chromium directly correlates with it being placed in the 4th period of the periodic table.

### Final Answer
Both the assertion and reason are correct, and the reason is a correct explanation of the assertion.