What is referred to as 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 is because protons and neutrons, which are collectively known as nucleons, determine the mass of the atom. Each proton and neutron contributes approximately one atomic mass unit to the total mass, making the mass number a crucial identifier of isotopes of an element.

Consequently, the mass number directly correlates to the atomic structure of an isotope and helps differentiate between different isotopes of the same element, which may have varying numbers of neutrons. While related terms such as the total number of particles in the nucleus can include protons and neutrons, the most precise definition of mass number specifically references the sum of protons and neutrons.