2022: SklogWiki celebrates 15 years on-line

Atomic mass units

From SklogWiki
Revision as of 12:33, 25 October 2007 by Nice and Tidy (talk | contribs)
Jump to: navigation, search

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 39.95
Xe 131.29
CO2 44.01
H2O (water) 18.02
N2 28.01