Talk:Mass excess

Rewrite
Noticing the tags, I attempted to briefly address them. I ought to qualify as an expert for introductory nuclear physics material. I hope the context is now more accessible to other readers, but I will leave them to remove those tags. There was also a significant error, which suggested that nuclei with more mass excess are more tightly bound, when in fact the opposite is true.

Below I paste the information I deleted and re-did, which was everything above the example. I mean no offense to anyone who worked on that, but in many cases, I find it easier to start anew, particularly when it comes to context, rather than attempting to do a lot of surgery. Please review and improve the new page, reverting or reinserting old material as you see fit. DAID (talk) 22:35, 31 July 2010 (UTC)

Former material
The mass excess of a nuclide is the difference between its actual mass and its mass number, i.e. nucleon-many atomic mass units. It is not the same as binding energy, although the concepts are related. 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.

Mass excess definition
I believe mass excess is the difference between the true mass M of an atom and the atomic number A (M - A), not the other way around! Thus, when the author gives the mass excess for uranium 236, he is wrong by a sign. Strangely, however, the mass excesses he provides for Kr-92, Ba-141, and a neutron ARE CORRECT, so he clearly ignored his own erroneous definition and did it the right way the second time. This inconsistency messes up his example calculation for the reaction's change in mass excess because the sign has been switched for reactants but not for products! Furthermore, when people compare the mass excesses of reactants and products in a reaction, most people find the difference in mass excess between the reactants and the products (mass excess of reactants - mass excess of products), not the other way around. This backward expression is similar to the backward way he defines mass excess in the beginning, and should be changed. Thus, I think the overall difference in mass excess between the reactants and the products of his example should be:

0.045563 − ( − 0.1334366) = 0.1789996 u

not,

− 0.1334366 − ( − 0.045563) = − 0.0878736 u

Could someone verify this? If I'm not mistaken, please correct this. Firth m (talk) 01:24, 3 March 2008 (UTC)

I agree with the mistake
Yes, the original author's definition of mass excess is backwards and should be equal to the actual mass minus the atomic mass, which makes the label "mass excess" make much more sense. I also agree that the given mass excesses for 92Kr, 141Ba, and the neutron are correct.

(see http://www.nndc.bnl.gov/masses/mass.mas03)

This should definitely be corrected in the future, as I looked to Wikipedia to learn the correct definition of mass excess and was erroneously turned down the wrong path in my calculation.

Annes72 (talk) 20:39, 17 April 2008 (UTC)annes72

A third on the mistake.
Added flag requesting an expert to correct the example. Sjodenenator (talk) 04:34, 21 September 2008 (UTC)

binding energy
The article states that the mass excess (difference between actual mass and mass number) for a nuclide is *solely* due to the binding energy between the nucleons. As i understand it, though this is partly true, the difference also arises as protons and neutrons have slightly different masses. The mass number just counts the number of nucleons, without taking into account the difference in their masses. I would appreciate it if someone could clear this up for me.

Thanks, telewatho 16:59, 19 May 2009 (UTC) —Preceding unsigned comment added by Telewatho (talk • contribs)


 * The mass number (A) is currently (could change any year now) based on, so equal protons and neutrons. It can be used to compare combinations of particles with the same total A and Z. So, yes, the mass excess of a neutron is different, but then again, the energy is available for weak decay, as it is for beta decay of the neutron.  Mass excess is best used to compute differences, where other changes will subtract out. Gah4 (talk) 20:15, 22 March 2017 (UTC)

Cyrillic 'c' for speed of light in Armenian article
I noticed that the Armenian article on "mass excess" uses the Cyrillic letter 'c' for the speed of light in a vacuum instead of the Latin letter 'c'. Is such use of Cyrillic letters that look like Latin ones normal and acceptable in Armenian texts? I would prefer that it be changed to the Latin letter. FYI: The letter 'C' for carbon in that article is the Latin letter. Sdiabhon Sdiamhon (talk) 17:19, 17 July 2020 (UTC)

Mass excess becomes positive from A = 216 on
The nuclide with the lowest mass among isobars of A = 215: 215At (mass excess = −1,255.756±6.799 keV, alpha half-life = 100 μs);

The nuclide with the lowest mass among isobars of A = 216: 216Rn (mass excess = 252.868±5.994 keV, alpha half-life = 45 μs).

Curiously, 216 is a cube number.

If 16O were used to define mass excess instead of 12C, then the border line lies between A = 177 and A = 178:

The nuclide with the lowest mass among isobars of A = 177: 177Hf (176.9994776×m(16O)/16);

The nuclide with the lowest mass among isobars of A = 178: 178Hf (178.0002737×m(16O)/16). 129.104.241.214 (talk) 21:58, 7 February 2024 (UTC)

Largest mass excess
If I got it right, 118Sn (117.9016066 u) has the largest mass excess among all nuclides. If 4He or 8Be were used to define mass excess instead of 12C, then the champion would be 140Ce (139.8144456×m(4He)/4): see here. Likewise, if we used 16O, then it would be 116Sn (115.93859232×m(16O)/16) that has the largest mass excess. 2A04:CEC0:1090:F6F2:916B:9970:62C3:CEC6 (talk) 22:44, 29 April 2024 (UTC)