Mass number (A) indicates the number of protons and neutrons an atom has in its nucleus. In other words, the number indicates the components that contribute to an atom’s mass. Since an electron is extremely light compared to a neutron or proton, it’s not considered when calculating mass number. Mass number doesn’t get a mention on the periodic table – the number below an element in the table is its atomic mass.

Like how atomic number defines an atom’s parent element, mass number indicates what isotope type the atom is. Isotopes are atoms of elements with variations among them. A particular element’s various isotopes won’t have identical mass numbers. But different elements and their isotopes could share the same mass number.