atommasseenhet

The atommasseenhet, often abbreviated as u, is a unit of mass used to express the mass of atoms and molecules. It is defined as one-twelfth of the mass of an unbound neutral atom of carbon-12 in its ground state. This standard is used internationally and is also known as the unified atomic mass unit (uamu).

The atommasseenhet is particularly useful in chemistry and physics because atomic and molecular masses are very small when expressed in kilograms. For instance, the mass of a single proton is approximately 1.67262192 × 10^-27 kilograms. Using the atommasseenhet, a proton's mass is approximately 1.007276 u, and a neutron's mass is approximately 1.008665 u. The carbon-12 atom itself has a mass of exactly 12 u by definition.

The value of the atommasseenhet in SI units is approximately 1.66053906660 × 10^-27 kilograms. This unit allows for convenient comparison of the masses of different isotopes and molecules. For example, the relative atomic mass of an element is the ratio of the average mass of atoms of that element to one-twelfth of the mass of an atom of carbon-12. Similarly, the relative molecular mass is the ratio of the average mass of molecules of that element to one-twelfth the mass of an atom of carbon-12. The atommasseenhet is also directly related to the concept of the mole and Avogadro's number.