A Level Physics Practice Exam

The nucleon number, also known as the mass number, is defined as the total number of protons and neutrons in the nucleus of an atom. This measurement is crucial because it reflects the total mass of the nucleus since protons and neutrons are much more massive than electrons and essentially constitute the bulk of the atom's mass.

In this context, protons are positively charged particles, while neutrons are neutral particles, and together they determine not only the mass of the nucleus but also its identity and stability. The nucleon number is fundamental in understanding various nuclear properties, such as isotopes and nuclear reactions, which are based on the arrangement and number of these particles.

Electrons, on the other hand, do not contribute to the nucleon number, as they are found outside the nucleus and their mass is negligible compared to that of protons and neutrons. Thus, counting only protons, only neutrons, or only electrons would not provide a complete picture of the atom's nucleon structure. Hence, the accurate definition incorporates both protons and neutrons, aligning perfectly with the chosen answer.