 |
The introduction to this article provides insufficient context for those unfamiliar with the subject. Please help improve the article with a good introductory style. (October 2009) |
 |
This article requires authentication or verification by an expert. Please assist in recruiting an expert or improve this article yourself. See the talk page for details. (September 2008) |
The mass excess of a nuclide is the difference between its actual mass and its mass number, i.e. nucleon-many atomic mass units.[1] It is not the same as binding energy, although the concepts are related.[2] It is a useful quantity when deciding whether a radioactive decay will occur and, if it does, how much energy will be released. Radioactive decay processes will only occur if the mass excess of the products is less than the mass excess of the parent nuclide.
The difference between the actual mass and the mass number arises from the mass-energy equivalence E=mc². When two nucleons come together, the potential energy between them due to the strong nuclear force is converted to mass. Qualitatively, if a nuclide has a high mass excess per nucleon, this indicates that the nucleus is tightly bound.
Example
Consider the nuclear fission of 236U into 92Kr 141Ba and three neutrons.
236U → 92Kr + 141Ba + 3 n
The mass number of 236U is 236 u (atomic mass units), but the actual mass is 236.045563 u, so the mass excess is + 0.045563 u. Calculated in the same manner, the mass excess for 92Kr, 141Ba, and a neutron are − 0.073843 u, − 0.085588 u and 0.0086648 u, respectively. The mass excess of the reactant is + 0.045563 u, and the mass excess of the products is − 0.073843 + ( − 0.085588) + 3 * (0.0086648) = − 0.1334366 The difference between reactants and products is 0.045563 − ( − 0.133436) = 0.1789996 u, which shows that the mass excess of the products is less than that of the reactants, and so the decay can occur.
The resulting difference in mass excess can be converted into energy using 1 u = 931.494 MeV/c², yielding 166 MeV.
References
- Kenneth S. Krane 1987. Introductory Nuclear Physics John Wiley & Sons ISBN 0-471-80553-X
- Paul A Tipler, Ralph A. Llewellyn 2004. Modern Physics WH Freeman and Co. ISBN 0-716-74345-0
Experimental atomic mass data compiled Nov. 2003
 |
This science article is a stub. You can help Wikipedia by expanding it. |
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
