What contributes to the mass of the nucleus of an atom?

The mass of the nucleus of an atom is predominantly determined by protons and neutrons, which are collectively known as nucleons. Protons carry a positive charge, while neutrons are electrically neutral. Together, they account for the vast majority of an atom's mass since electrons, which are significantly lighter and occupy the space around the nucleus, contribute negligibly to the overall mass.

Therefore, the correct answer highlights that only protons and neutrons contribute to the nuclear mass, affirming their role as the primary components of atomic mass while excluding electrons from consideration due to their minimal contribution. Understanding this concept is essential in nuclear physics and helps clarify the structure and behavior of atoms in various contexts, including radiation safety.