what is atomic mass unit

An atomic mass unit (AMU) is a unit for expressing the mass of atoms, molecules, or subatomic particles. It is defined as one-twelfth of the mass of an atom of carbon-12, the most abundant isotope of carbon, or 1.660538921 × 10 −24 gram. The atomic mass constant, denoted mu, is defined identically, giving mu = m(12C)/12 = 1 Da. The AMU is commonly used in physics and chemistry to express the mass of atomic-scale objects, such as atoms, molecules, and elementary particles, both for discrete instances and multiple types of ensemble averages. The mass of any isotope of any element is expressed in AMU or Da. The unified atomic mass unit (u) is generally considered a synonym for AMU and is a physical constant accepted for use in the International System of Units (SI) measurement system.