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Difference between revisions of "Atomic mass units"

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Line 26: Line 26:
| C || 12.0107
| C || 12.0107
| =CH || 13.01864
| -CH<sub>2</sub>- || 14.02658
| -CH<sub>2</sub>- || 14.02658

Revision as of 17:51, 29 October 2007

40px-Stop hand nuvola.svg.png This page contains numerical values and/or equations. If you intend to use ANY of the numbers or equations found in SklogWiki in any way, you MUST take them from the original published article or book, and cite the relevant source accordingly.

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 39.948
C 12.0107
=CH 13.01864
-CH2- 14.02658
-CH3 15.03452
Xe 131.293
CO2 44.0095
H 1.00794
H2O (water) 18.02
N2 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