What is the nucleon number of an atom?

The nucleon number, also known as atomic mass number, is defined as the total number of nucleons in the nucleus of an atom. Nucleons include both protons and neutrons. Protons carry a positive charge, while neutrons have no charge, but both contribute to the overall mass of the atom and determine its identity as a particular element.

For example, carbon has a nucleon number of 12, which means it has 6 protons and 6 neutrons. The nucleon number is essential for understanding the stability and isotope variations of an element, since isotopes of an element will have the same number of protons but different numbers of neutrons, leading to different nucleon numbers.

Thus, the correct choice reflects the combined count of both protons and neutrons, making it clear that the nucleon number is crucial in describing the atomic structure.