An atom that is identified uniquely by the number of protons and neutrons in its nucleus is called a

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

Answer: (B)

A nuclide is a specific nucleus defined by its number of protons and neutrons. This term captures the exact identity of the atom’s nucleus, since knowing Z (the number of protons) and N (the number of neutrons) fixes the nucleus uniquely. The idea here is that while atoms of the same element can vary in their neutron count, those variations are called isotopes, which are different nuclides of the same element. The atomic number only tells you how many protons are present and thus which element it is, not the neutrons. The atomic mass unit is simply a unit of mass, not a name for the nucleus.