How does the mass number of an atom differ from its atomic number?

The atomic number and mass number are distinct properties used to define the composition of an atom.

• Atomic Number: This is the number of protons found in the nucleus of an atom. It is a characteristic value for each chemical element and determines the element's position in the periodic table. Because atoms are electrically neutral, the atomic number also indicates the number of electrons surrounding the nucleus.123
• Mass Number: This is the total sum of the number of protons and neutrons in the nucleus. While the atomic number is constant for all atoms of a specific element, the mass number can vary among isotopes of that element because they contain different numbers of neutrons.124

In summary, the atomic number defines the identity of the element, while the mass number reflects the relative weight of a specific atom based on its total number of nucleons.2