Assertion: Cu, Ag and Au lie in Group 11 of the periodic table. Hence their atomic radii are in the order `Cu ltAgltAu`
Reason: In any group iof the transition elements, the atomic radii increase as we move down the group.

Options

A. If both (A) and (R) are correct and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct, but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect but (R) is correct.

Explanation

To solve the question, we need to analyze both the Assertion and the Reason provided.

### Step 1: Analyze the Assertion
The assertion states that "Cu, Ag, and Au lie in Group 11 of the periodic table. Hence their atomic radii are in the order Cu < Ag < Au."

- **Group 11 Elements**: Copper (Cu), Silver (Ag), and Gold (Au) are indeed located in Group 11 of the periodic table.
- **Atomic Radii Order**: As we move down the group from Cu to Ag to Au, we expect the atomic radii to increase. This is because with each successive element, an additional electron shell is added, which increases the size of the atom.

### Step 2: Analyze the Atomic Configurations
- **Copper (Cu)**: Atomic configuration is [Ar] 3d^10 4s^1.
- **Silver (Ag)**: Atomic configuration is [Kr] 4d^10 5s^1.
- **Gold (Au)**: Atomic configuration is [Xe] 4f^14 5d^10 6s^1.

From the configurations, we can see:
- Cu has its outermost electron in the 4th shell.
- Ag has its outermost electron in the 5th shell.
- Au has its outermost electron in the 6th shell.

### Step 3: Confirm the Assertion
Since the atomic radii increase as we move from Cu to Ag to Au due to the addition of electron shells, the assertion is **true**.

### Step 4: Analyze the Reason
The reason states that "In any group of the transition elements, the atomic radii increase as we move down the group."

- This statement is also **true**. As we move down a group in the periodic table, the number of electron shells increases, which leads to an increase in atomic size.

### Step 5: Conclusion
Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the correct option is that both A and R are correct, and R is the correct explanation of A.

### Final Answer
Both Assertion (A) and Reason (R) are true, and R is the correct explanation of A.

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