The mass number of an atom is the total number of neutrons and protons in its nucleus. It is a fundamental property of an atom that determines its place in the periodic table. The mass number is often used to identify and distinguish different isotopes of an element, which have the same number of protons but different numbers of neutrons.

The mass number of an atom is often written as a superscript to the left of the element's chemical symbol. For example, the mass number of carbon-12 is 12, which indicates that the nucleus of a carbon-12 atom contains 6 protons and 6 neutrons.

The mass number of an atom can be calculated by adding the number of protons and neutrons in its nucleus. For example, the mass number of carbon-12 is calculated as follows:

Mass number = number of protons + number of neutrons

Mass number = 6 + 6

Mass number = 12

The mass number of an atom is a very important property that can be used to identify and distinguish different isotopes of an element. It also provides insight into the structure and composition of the atomic nucleus.