The correct option is **C. Number of neutrons**. ### Detailed Explanation: 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 that while they belong to the same element (and thus have the same atomic number), they differ in their atomic mass. 2. **Protons and Electrons**: - **Number of Protons (Option A)**: The number of protons in an atom defines the element itself. For example, all carbon atoms have 6 protons. Therefore, this characteristic does not distinguish isotopes of the same element, as isotopes have the same number of protons. - **Number of Electrons (Option B)**: In a neutral atom, the number of electrons equals the number of protons. While ions (charged atoms) can have a different number of electrons, isotopes are not defined by their electron count. Thus, this option is also incorrect. 3. **Neutrons**: - **Number of Neutrons (Option C)**: This is the key distinguishing feature of isotopes. For example, carbon-12 has 6 neutrons (6 protons + 6 neutrons = atomic mass of 12), while carbon-14 has 8 neutrons (6 protons + 8 neutrons = atomic mass of 14). The difference in the number of neutrons leads to different atomic masses and some differences in nuclear stability and behavior. 4. **Atomic Mass Number**: - **Atomic Mass Number (Option D)**: The atomic mass number is the total number of protons and neutrons in an atom. While it is a result of the number of neutrons (and protons), it does not directly distinguish isotopes. For example, both carbon-12 and carbon-14 have different atomic mass numbers (12 and 14, respectively), but it is the number of neutrons that directly accounts for this difference. ### Summary of Why Other Options are Incorrect: - **A. Number of Protons**: All isotopes of an element have the same number of protons. - **B. Number of Electrons**: The number of electrons can vary in ions but does not distinguish isotopes. - **D. Atomic Mass Number**: While it reflects the difference in isotopes, it is derived from the number of neutrons and protons, not a direct characteristic. ### Common Pitfalls: - Confusing isotopes with ions: Isotopes differ in neutrons, while ions differ in electrons. - Misunderstanding atomic mass: Atomic mass is not a fundamental property but a calculated value based on protons and neutrons. ### Revision Summary: - Isotopes are variants of the same element with the same number of protons but different numbers of neutrons. - The number of neutrons is the key characteristic that distinguishes isotopes. - Atomic mass number is a result of the number of protons and neutrons, not a distinguishing feature. - Protons and electrons do not vary among isotopes of the same element.