Which term describes the total number of protons and neutrons in an atomic nucleus?

The correct term that describes the total number of protons and neutrons in an atomic nucleus is mass number. This concept is fundamental in nuclear chemistry and physics, as it provides a measure of the size of an atomic nucleus. The mass number is calculated by simply adding the number of protons (which defines the element) and the number of neutrons (which can vary in isotopes of the same element) together.

For example, in a carbon atom, which typically has 6 protons and 6 neutrons, the mass number would be 12. This value is crucial in understanding the stability and behavior of different isotopes, as well as in nuclear reactions.

While the other options relate to atomic structure, they do not accurately describe this specific characteristic. Mass defect refers to the difference between the mass of a nucleus and the sum of the masses of its individual protons and neutrons, atomic mass refers to the weighted average mass of an element’s isotopes, and atomic weight is the ratio of the average mass of the atoms of an element to one twelfth of the mass of a carbon-12 atom. Each of these terms has distinct meanings and applications in the field of chemistry and physics.