The atomic mass is the microscopic mass of an element in atomic mass units. Also known as the average mass, or atomic weight, it is roughly the sum of protons (the atomic number) and neutrons in an atomic nucleus. The same element may have various atomic masses, also known as isotopes (for example 235U has a different atomic mass than 238U). These isotopes will vary in the number of neutrons not protons. Electrons orbit around the nucleus and their mass is much smaller, therefore, isn't considered as part of the atom's mass.
Below, all the numbers listed beside each element is the atomic mass for the most common isotope. Choose a clickable element in the table below to learn more about it and its role in energy and society.