What defines an isotope?

• A. A variation of an element based on the amount of neutrons in the atom
• B. A variation of an element based on the number of protons
• C. A charged version of the element
• D. A different element

Answer: (A)

Isotopes are atoms of the same element that differ in the number of neutrons in the nucleus, while the number of protons stays the same. Because protons define the element, all isotopes have the same identity and chemical behavior, but their masses and nuclear stability can differ as neutrons vary. For example, carbon atoms can have 6 protons but different numbers of neutrons (like 6, 7, or 8), giving carbon-12, carbon-13, or carbon-14. That’s why the description focusing on neutron variation best defines isotopes. The other ideas point to changing the element (different number of protons) or to ions (charged atoms) or to a different element altogether.