An atom that has the same number of protons as other atoms of the same element but a different number of neutrons is called an

• A. Nuclide
• B. Atomic Number
• C. Mass Number
• D. Isotope

Answer: (D)

Isotopes are atoms of the same element that differ in the number of neutrons. The identity of an element is set by the number of protons (the atomic number), which stays the same for all atoms of that element. When neutrons vary while protons stay the same, the mass number A = Z + N changes, giving different isotopes with the same chemical behavior but different masses and nuclear properties. Nuclide is a general term for any nucleus defined by its Z and N, so isotopes are nuclides, but the word isotope specifically highlights the variation in neutron number within the same element. The mass number by itself is just the total count of protons and neutrons and does not name the type of atom, and the atomic number accounts for protons only, not neutron variation.