Which of the following describes the mass number?

The mass number of an atom is defined as the total number of protons and neutrons present in its nucleus. This is significant because protons and neutrons are the particles that contribute significantly to the mass of the atom, whereas electrons have a negligible mass compared to these particles.

Understanding mass number is crucial for distinguishing between different isotopes of an element, which have the same number of protons but differing numbers of neutrons, thereby affecting their mass number. In contrast, the number of protons alone determines the element itself, while the number of neutrons (not just alone) adds to the mass number but does not define it by itself. This understanding helps in predicting the behavior of elements in chemical reactions and their stability.