Concept:Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Explanation:An element is identified by its atomic number, which equals the number of protons.
Isotopes of the same element always have the same number of protons.
In a neutral atom, the number of electrons also stays the same for isotopes.
The only difference is the number of neutrons in the nucleus.
This difference changes the mass number but not the chemical identity.
So, an element and its isotope differ only in neutron number.
Answer:D. neutrons