Q.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.

Concept: Chemical behaviour is determined by electronic configuration, which depends on the number of electrons (equal to the atomic number). Isotopes differ only in neutron count, not in proton or electron count.

Reasoning:

1. Isotopes of an element have the same atomic number $Z$ (same number of protons) but different mass numbers $A$ (different numbers of neutrons).

2. In a neutral atom, the number of electrons equals the number of protons. Since isotopes have identical $Z$, they possess the same number and arrangement of electrons.

3. Chemical properties—bonding, reactivity, valence—are governed entirely by the electron configuration, particularly the valence electrons. …

Chemical behavior springs from how atoms interact with one another, and those interactions are governed entirely by electrons—specifically, the valence electrons that participate in bonding. The nucleus, hidden deep inside and thousands of times smaller than the electron cloud, plays almost no direct role in chemistry. It holds the positive charge that attracts electrons, but once that electron configuration is set, the chemistry is set.

Isotopes are atoms of the same element with different numbers of neutrons. Because they have the same atomic number, they have the same number of protons and, in a neutral atom, the same number of electrons. That electron count and arrangement is what matters for chemistry.

Let's examine each statement carefully.

Evaluating the Assertion

1. What defines an element? The number of protons (atomic number $Z$). Carbon is carbon because it has 6 protons, oxygen is oxygen because it has 8.

2. What are isotopes? Atoms with the same $Z$ but different mass numbers $A$ (because they have different numbers of neutrons $N = A - Z$). For example, $^{12}\text{C}$, $^{13}\text{C}$, and $^{14}\text{C}$ all have 6 protons and 6 electrons, but 6, 7, and 8 neutrons respectively.

3. Do isotopes have the same electron configuration? Yes. A neutral $^{12}\text{C}$ atom has 6 electrons arranged as $1s^2 \, 2s^2 \, 2p^2$. So does $^{13}\text{C}$ and $^{14}\text{C}$. The neutron count does not affect how electrons fill orbitals.

4. Do they show the same chemical behavior? Yes. All three carbon isotopes form the same bonds (four covalent bonds in organic molecules), react with oxygen to form CO₂, and participate in identical reactions. The reaction mechanisms, products, and equilibria are the same.

The assertion is true.

Evaluating the Reason …