Mass Number

Name:
Mass Number
Detailed information:
The number of neutrons and protons in the nucleus of an atom. Thus, the mass number of normal helium is 4; of carbon, 12; of oxygen, 16; and of uranium 238. A given atom is characterized by its atomic number, equivalent to the number of protons, which give it its charge and thus determine the kind of element, and by its mass number, which includes the neutrons that make up the remainder of its mass. Helium has 2 protons and 2 neutrons (mass number 4 and atomic number 2). Protons and neutrons each have very close to unit mass, and since the mass change associated with binding the particles together in the nucleus is very small, the mass number is always within 1/10th unit of the atomic weight of the nuclide.See Mass; Mass Defect; Atomic Weight.