Which term refers to a distinct species of atom defined by the total number of protons and neutrons in its nucleus?

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

Answer: (D)

The main idea is recognizing how nuclear identity is defined. A nuclide is a specific nucleus characterized by the exact numbers of protons and neutrons it contains. Each unique pairing of Z (protons) and N (neutrons) defines a distinct nuclide, so you get a unique species of atom. The total number of nucleons, A = Z + N, is a handy way to describe mass, but the naming of the species itself is based on that specific Z and N combination, i.e., the nuclide. Isotopes are nuclides of the same element that differ in neutron number, which is why they’re a subset of nuclides like carbon-12 and carbon-14. Atomic number tells you the element (protons only), and the atomic mass unit is a mass unit, not a classification of nucleus. So the term that fits the description is nuclide.