What is the total number of protons and neutrons in an atom called?

The total number of protons and neutrons in an atom is referred to as the mass number. This value is significant because it provides insight into the atomic structure of an element. The mass number helps distinguish between different isotopes of the same element, which have the same number of protons but different numbers of neutrons.

For instance, carbon typically has an atomic number of 6 (indicating 6 protons) and can have isotopes such as carbon-12 and carbon-14, where the mass number differs due to the different counts of neutrons. Thus, the mass number is crucial for understanding atomic behavior in both chemical reactions and nuclear processes.

In contrast, the atomic number pertains only to the number of protons in an atom, atomic weight refers to the average mass of an atom's isotopes (accounting for their natural abundance), and molecular mass pertains to the mass of a molecule, which includes the combined masses of all atoms within that molecule.