Assertion (A): Mendeleev left the gap under aluminium and silicon and called these Eka-aluminium and Eka-silicon, respectively.
Reason (R): Dobereiner arranged elements on the basis of increasing atomic number.

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)**:
- Mendeleev, a prominent chemist, left gaps in his periodic table for elements that had not yet been discovered. Specifically, he left a gap below aluminum and silicon, predicting the existence of new elements he referred to as Eka-aluminium and Eka-silicon.
- Later discoveries confirmed his predictions, with gallium (Ga) being the element below aluminum and germanium (Ge) being the element below silicon.

2. **Analyzing the Reason (R)**:
- Dobereiner was another chemist who contributed to the classification of elements. However, he arranged elements based on their atomic masses, not atomic numbers. His work involved grouping elements into triads based on similar properties and their atomic masses.
- The statement in the reason claims that Dobereiner arranged elements based on increasing atomic number, which is incorrect. The modern periodic table is arranged by increasing atomic number, but Dobereiner's method was based on atomic mass.

3. **Conclusion**:
- Since the assertion (A) is true (Mendeleev did leave gaps and predicted elements), but the reason (R) is false (Dobereiner arranged elements by atomic mass, not atomic number), we conclude that the correct relationship is: Assertion is true, but Reason is false.

**Final Answer**: The assertion is true, but the reason is false.

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