Atomic mass units

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The atomic mass unit (AMU) is defined as the twelfth part of the mass of a C_{12} atom. Its value (in SI units, CODATA value) is

1.66054\times10^{-27} kilograms

Hence, it is approximately equal to the mass of a proton. Elements are assigned masses in these units which are not integer numbers. This is because neutrons and protons have slightly different masses, because nuclear binding forces cause the total mass of a nucleus to be less than that of the sum of its constituents, and, mainly, because of the appearance of natural isotopes. Thus, Potassium has a mass of 39.10 mainly because of natural K^{40} isotopes.

Here is a list of masses of elements and compounds in atomic mass units:

Compound/element mass in amu
Ar (argon) 39.948
C 12.0107
=CH 13.01864
-CH2- 14.02658
-CH3 15.03452
Xe 131.293
CO2 (carbon dioxide) 44.0095
H 1.00794
H2O (water) 18.02
N2 (nitrogen) 28.01

Note that sometimes the atomic mass unit is also known as the Dalton (Da), especially when dealing with macromolecules such as proteins.


  1. IUPAC Periodic Table