Assertion (A): All isotopes of a given element show the same type of chemical behaviour.
Reason (R): The chemical properties of an atom are controlled by the number of electrons in the atom.

1. Both (A) and (R) are true and (R) is the correct explanation of (A).
2. Both (A) and (R) are true but (R) is not the correct explanation of (A).
3. (A) is true but (R) is false.
4. (A) is false but (R) is true.

Hint Chemical property depends on the number of electrons
Atoms with the same atomic number but with different atomic masses are called isotopes. In isotopes, a number of electrons are the same and these electrons are responsible for their chemical behaviour. 
Hence, isotopes exhibit similar chemical properties