(AMU) is equal to the mass of one-twelfth of a carbon-12 atom (6 protons + 6 neutrons (+ 6 electrons)), a 'very' little less than 1.66 x 10^-24 gram.

See also Mass Number and Atomic Mass.

The atomic mass is the mass of the naturally occurring mixture of isotopes of an element.

See also Mass Number, Atomic Mass Unit and Isotope.

The mass number is the total number of protons and neutrons (also total nucleon) in the nucleus.

See also Atomic Mass Unit, Atomic Mass.