The mass number is equal to the sum of the number of protons and neutrons located within the nucleus of an atom.
An atom is made of three subatomic particles. The subatomic particles that make up an atom are protons, neutrons, and electrons.
Each subatomic particle has a different charge. Protons are positively charged. Neutrons are neutrally charge.
Electrons are negatively charged. Subatomic particles are also found in distinct locations within an atom. As mentioned above, protons and neutrons are located in the central nucleus of an atom. Electrons are thought to travel around the nucleus in either electron clouds or orbitals.
Finally, each type of subatomic particle has a characteristic mass. Protons and neutrons are each accredited with having a mass of 1.0 amu (atomic mass unit). Electrons have a mass of only .00054 amu. Thus, the mass from electrons is usually not represented in the mass of an atom. Therefore, the mass number of an atom is essentially the mass of that atom.