Assertion (A) : The isotopes are the atoms of the same element with different atomic masses. They require different positions in the periodic table.
Reason (R) : Variable number of neutrons in the nuclei of the atoms of same elements leads to the different atomic masses.

Options

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

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step 1: Understand the Assertion (A)
The assertion states: "The isotopes are the atoms of the same element with different atomic masses. They require different positions in the periodic table."

- Isotopes are indeed atoms of the same element that have different atomic masses due to a different number of neutrons.
- However, isotopes of the same element do not occupy different positions in the periodic table; they are placed in the same position because they have the same atomic number (number of protons).

**Conclusion for Assertion (A)**: The first part is true, but the second part is false. Therefore, the assertion is **false**.

### Step 2: Understand the Reason (R)
The reason states: "Variable number of neutrons in the nuclei of the atoms of the same elements leads to the different atomic masses."

- This statement is true. The difference in the number of neutrons among isotopes of the same element results in different atomic masses.

**Conclusion for Reason (R)**: The reason is **true**.

### Final Conclusion
- Assertion (A) is **false**.
- Reason (R) is **true**.

Thus, the correct answer is that the assertion is false, but the reason is true.