Talk:Atomic mass

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Persistent Confusion on Relative Atomic Mass[edit]

I have once again corrected the persistent confusion on relative atomic mass. The most important point to remember is that relative atomic mass is practically an antonym of atomic mass. Relative atomic mass is a weighted average and atomic mass is not. I know this is very confusing but please read definitions carefully and make sure you understand the differences.

Correct statement:

Relative atomic mass is a synonym of atomic weight.

Incorrect Statement:

Relative atomic mass and relative isotopic mass are essentially the same.

Why: The relative atomic mass is a weighted average. The relative isotopic mass is not.

Incorrect Statement:

Relative atomic mass and atomic mass are essentially the same.

Why: The relative atomic mass is a weighted average. The atomic mass is not.

Incorrect statement:

The mass defect of all atomic masses above C12 are positive.

Why: They are in fact mostly negative and only become positive at high Z and low Z. The error here is in the confusion between mass defect of atoms and the atomic weights (or more precisely the standard atomic weights) that mostly have masses slightly above the nominal atomic weight. This trend in the decimal places of the atomic weights has to do the relative prevalence of heavier isotopes at higher Z and has little to do with mass defect due to nuclear binding energies.

--Nick Y. (talk) 18:33, 22 September 2009 (UTC)[reply]

I find the objections above all correct, and hopefully they have all been fixed in the present version of the article. S B H arris 23:49, 10 October 2014 (UTC)[reply]

The section on mass number deviation is an overly-broad generalization.[edit]

"The amount that the ratio of atomic masses to mass number deviates from 1 is as follows: the deviation starts positive at hydrogen-1, becomes negative until a minimum is reached at iron-56, iron-58 and nickel-62, then increases to positive values in the heavy isotopes, with increasing atomic number. This corresponds to the fact that nuclear fission in an element heavier than zirconium produces energy, and fission in any element lighter than niobium requires energy. On the other hand, nuclear fusion reactions: fusion of two atoms of an element lighter than scandium produces energy, whereas fusion in elements heavier than calcium requires energy."

This is a generalization that is sort of true most of the time, but shouldn't be stated without qualification. I would suggest it not be stated at all. Its most glaring defect is when it comes to the spike in binding energy for He-4. If this statement were true, we could fuse two He-4 nuclei to produce Be-8 with a release of energy. I don't believe that happens, and it isn't what this graph would predict. In fact, based on another Wikipedia article, I believe the reverse happens: Be-8 decays almost immediately into two He-4's with the release of energy.

A better thing to do here might be to talk about why fission of U-235 produces energy and why fusion of deuterium nuclei produces energy, referring to the graph. I should point out that I am not an expert in nuclear, or any other kind, of physics. I'm just an interested amateur. Otherwise I would be happy to write that text. — Preceding unsigned comment added by Stuart.soloway (talk • contribs) 21:35, 4 April 2014 (UTC)[reply]

You're certainly right about the fusion energy bottleneck created by the stability of He-4 so it cannot simply fuse to Be-8. If it weren't for the triple alpha process in stars that gets around that, we wouldn't be here! I'll see if I can generalize the section above so it's not strictly wrong in that regard. S B H arris 01:24, 10 October 2014 (UTC)[reply]

Stylized lithium-7 atom?[edit]

The picture of the Stylized lithium-7 atom has (3) electrons each at equal distance orbit from the nucleus, which seems misleading. Is there a case in real life where you could force all (3) electrons in lithium-7 to have equal energy and be stable in a single electron shell? Even in the bohr model, you'd expect (2) shells, not a single shell. A picture of a Quantum model of lithium-7 I think would be better. Or at least a picture showing (2) electron shells... Goslackware (talk) 17:27, 4 October 2023 (UTC)[reply]

Is there a case in real life where you could force all (3) electrons in lithium-7 to have equal energy and be stable in a single electron shell?

The Pauli exclusion principle says definitely no. However the electronic structure of the atom is not relevant to this particular article, and a more accurate diagram might get in the way of the intended meaning. FChlo (talk) 01:27, 16 January 2024 (UTC)[reply]