Concept:Isotopes are atoms of the same element that differ only in the number of neutrons they contain.
Explanation:The identity of an element is fixed by its atomic number, which is the number of protons in its nucleus.
Isotopes of an element therefore always have the same number of protons.
However, they contain different numbers of neutrons, which changes their mass number but not their chemical identity.
In a neutral atom, the number of electrons equals the number of protons, so isotopes also have the same number of electrons.
Thus, the defining feature of isotopes is “same protons, different neutrons.”
Answer:A. The same number of protons but different numbers of neutrons.