Atomic Mass Unit
Definition
An atomic mass unit (symbol: u) is a unit of mass used to express atomic and molecular weights. It is defined as one-twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, which is approximately 1.66 x 10^-27 kilograms. Atomic structure and chemical bonding are two areas where the atomic mass unit is commonly used.
History
The concept of an atomic mass unit has been around since the early 19th century, when John Dalton proposed his atomic theory. Dalton's theory suggested that each element consists of atoms of a single, unique type, and that these atoms can combine to form chemical compounds. However, it was not until the 20th century that the atomic mass unit was formally defined.
Modern Definition
The atomic mass unit is currently defined by the International Committee for Weights and Measures (CIPM) as 1/12th of the mass of a carbon-12 atom. This definition was adopted in 1961, replacing the previous definition based on oxygen-16. The change was made because it was discovered that the oxygen-16 scale was slightly off due to the presence of multiple isotopes.
Use in Science
In the field of chemistry, the atomic mass unit is used to express the masses of atoms and molecules. For example, the atomic mass of hydrogen is approximately 1 u, while the atomic mass of oxygen is approximately 16 u. This allows scientists to easily compare the masses of different atoms and molecules.
In physics, the atomic mass unit is used in nuclear physics to express the masses of particles such as protons and neutrons. For example, the mass of a proton is approximately 1 u, while the mass of a neutron is slightly more than 1 u.
Relation to Other Units
The atomic mass unit is related to other units of mass through the physical constant known as the Avogadro constant. The Avogadro constant, denoted by N_A, is the number of entities (such as atoms or molecules) in one mole of a substance. It is defined as exactly 6.02214076×10^23 entities per mole.
This constant allows us to convert between the atomic mass unit and the kilogram. Specifically, 1 u is approximately equal to 1.660539040×10^-27 kg. This conversion factor is used in many calculations in chemistry and physics.