Chemistry glossary Definition of the atomic mass units (amu) history and examples
Atomic mass units or AMU is a physical constant equal to one twelfth of the mass of an unbound atom of carbon -12. It is a unit of mass used to express atomic and molecular masses . When the mass is expressed in amu, it roughly reflecting the sum of the number of protons and neutrons in the atomic nucleus (electrons have so much less mass that they are assumed to have negligible effect).
The unit symbol is u (unified atomic mass unit) or Da (Dalton), although amu can still be used.
1 u = 1 Da = 1 uma (in modern use) = 1 g / mol
Also known as:
It is also known as unified atomic mass unit (U), Dalton (Da), universal mass unit, either AMU or AMU is an acceptable acronym for atomic mass unit
“Unified atomic mass unit” is an accepted physical constant for use in the SI measurement system. It replaces “atomic mass unit” (without the unified part) and it is the mass of a nucleon (either a proton or a neutron) of a carbon-12 neutral atom in its ground state. Technically, amu was the oxygen-based unit 16 until 1961, when it was redefined on the basis of carbon 12. Today, people use the expression “atomic mass unit” , but what they really mean is “unified atomic mass unit”.
A unified atomic mass unit is equal to:
- 1.66 yoctograms
- 1.66053904020 x 10 -27 kg
- 1.66053904020 x 10 -24 g
- 931.49409511 MeV / c 2
- 1822.8839 m e
History of the atomic mass unit
John Dalton first suggested a way to express the relative atomic mass in 1803. He proposed the use of hydrogen-1 (protium). Wilhelm Ostwald suggested that the relative atomic mass would be better if expressed in terms of 1/16 of the mass of oxygen. When the existence of isotopes was discovered in the year 1912 and isotopic oxygen in 1929, the definition based on oxygen became confused.
Some scientists used an AMU based on the natural abundance of oxygen, while others used an AMU based on the isotope of oxygen 16. So, in 1961, it was decided to use the carbon 12 as the basis of the unit (to avoid any confusion with a unit defined by oxygen). The new unit has received the u symbol to replace amu, plus some scientists have called the new unit a Dalton. However, u and Da were not universally adopted. Many scientists continued to use amu, simply acknowledging that it was now based on carbon rather than oxygen. Currently, the values expressed in u, AMU, amu and Da all describe the exact same measurement.
Examples of values expressed in atomic mass units
- A hydrogen-1 atom has a mass of 1.007 u (or Da or amu).
- A carbon-12 atom is defined as having a mass of 12 u.
- The largest protein, titin, has a mass of 3 x 10 6 Da.
- AMU is used to differentiate isotopes. An atom of U-235, for example, has a lower AMU than that of U-238 because they differ in the number of neutrons in the atom.
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