Isotopes differ in

Options

A. the number of protons
B. the valency electrons
C. the chemical activity
D. the number of neutrons

Answer

**Step-by-Step Solution:**

1. **Understanding Isotopes**: Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.

2. **Identifying the Components**:
- **Atomic Number (Z)**: This is the number of protons in the nucleus of an atom. For isotopes of the same element, this number remains constant.
- **Mass Number (A)**: This is the total number of protons and neutrons in the nucleus. Since isotopes differ in the number of neutrons, their mass numbers will differ.

3. **Analyzing the Options**:
- **Option A (Number of Protons)**: This is incorrect because isotopes have the same number of protons.
- **Option B (Valency Electrons)**: This is also incorrect as the number of valency electrons is determined by the number of protons (which is the same for isotopes).
- **Option C (Chemical Activity)**: This is not a distinguishing factor for isotopes since chemical activity is primarily determined by the number of valence electrons, which remains unchanged.
- **Option D (Number of Neutrons)**: This is correct because isotopes differ in the number of neutrons they contain.

4. **Conclusion**: The correct answer is Option D: isotopes differ in the number of neutrons.