Atomic Mass Unit - Relationship To SI

Relationship To SI

The definition of the mole, an SI base unit, was accepted by the CGPM in 1971 as:

1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12; its symbol is "mol".
2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

The definition of the mole also determines the value of the universal constant that relates the number of entities to amount of substance for any sample. This constant is called the Avogadro constant, symbol N_A or L, and is equal to 6.02214129(27)×1023 entities per mole.

Given that the unified atomic mass unit is one twelfth the mass of one atom of carbon-12, meaning the mass of such an atom is 12 u, it follows that there are N_A atoms of carbon-12 in 0.012 kg of carbon-12. This can be expressed mathematically as

N_A (12 u) = 0.012 kg/mol, or

N_A u = 0.001 kg/mol

Masses of proteins are often expressed in daltons. For example, a protein with a molecular weight of 64000 g mol−1 has a mass of 64000 daltons or 64 kDa in short.