Isotope refers to?

• A. Atoms with the same number of protons but different numbers of neutrons.
• B. Atoms with different numbers of protons and neutrons.
• C. Atoms with the same neutrons but different electrons.
• D. Isotopes refer to charged particles in the nucleus only.

Answer: (A)

Isotopes are atoms with the same number of protons but different numbers of neutrons. The identity of an element is determined by its proton count, so isotopes share the same element but vary in mass because the neutrons differ. The electrons can change without changing the isotope, and that’s why isotopes have nearly identical chemical behavior. The other statements don’t define isotopes: changing both protons and neutrons would make a different element; changing neutrons while keeping electrons the same defines isotopes, not changes to electron count; and isotopes aren’t about charged particles in the nucleus.