What quantity is equal to the mass number of an atom?

The mass number of an atom is defined as the total number of protons and neutrons present in its nucleus. This quantity reflects the atom's overall mass, as both protons and neutrons contribute significantly to the atomic mass, while electrons have negligible mass. Protons carry a positive charge, and neutrons are neutral, together making up the nucleus of the atom and accounting for the bulk of its mass.

Understanding this concept is crucial in fields like chemistry and physics, where the mass number plays a significant role in identifying isotopes and understanding atomic behavior. By recognizing that the mass number is the sum of protons and neutrons, one gains insight into the structural makeup of different atoms and how they interact in various chemical reactions and physical processes.