Assertion (A): Mendeleev arranged element in horizontal rows and vertical columns.
Reason (R): Mendeleev ignored the order of atomic weight thinking that the atomic measurements might be incorrect.

Options

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

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion (A):**
- The assertion states that "Mendeleev arranged elements in horizontal rows and vertical columns."
- This is true; Mendeleev's periodic table was organized into rows (called periods) and columns (called groups or families).

2. **Understanding the Reason (R):**
- The reason provided is that "Mendeleev ignored the order of atomic weight thinking that the atomic measurements might be incorrect."
- This statement is also true. Mendeleev did recognize that some atomic weights might not be accurate, and he prioritized the chemical properties of elements over their atomic weights when arranging them.

3. **Analyzing the Relationship Between A and R:**
- Since both the assertion and the reason are true, we need to determine if the reason correctly explains the assertion.
- Mendeleev's decision to arrange elements based on their properties rather than strictly by atomic weight is indeed the reason he organized them into horizontal rows and vertical columns.

4. **Conclusion:**
- Both the assertion (A) and the reason (R) are true, and R is the correct explanation of A. Therefore, the correct answer is that both A and R are true, and R is the correct explanation of A.

**Final Answer:** Both A and R are true, and R is the correct explanation of A.

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