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Atomic weight of Br-79 is 79.641 if you add the masses of protons and neutrons. However, in periodic table, it is less than the value given here. How is the difference arrived at for all the elements listed in the periodic table?

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    $\begingroup$ This is really a question about nuclear physics, not chemistry. See physics.stackexchange.com/q/60802. $\endgroup$ – f'' Aug 8 '16 at 1:43
  • $\begingroup$ What do you mean by "arrived for"? Mass of nuclei can be measured but it's not chemistry. $\endgroup$ – Mithoron Aug 8 '16 at 1:51
  • $\begingroup$ @Mithoron It means atomic weights mentioned in periodic table are borrowed from the works of nuclear physics. $\endgroup$ – Mathivanan Palraj Aug 8 '16 at 6:19
  • $\begingroup$ What do you mean by the atomic weight of Br-79 being 79.641? It's clearly not an atomic mass but a molar mass $\endgroup$ – szentsas Aug 8 '16 at 14:35
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Yes, there is the Semi-empirical mass formula which will give you the binding energy/mass of the nucleus.

Once you subtract the binding energy/mass from that of the nucleons, you have the mass of the nucleus. Then you should add the mass of the electrons. Finally, in principle at least, you would subtract the binding energy/mass of the electrons (in other words the ionization energies of the electrons), but this is a very minor correction.

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  • $\begingroup$ Thanks. However, if you could provide the calculations for Br-79 it will be useful to understand how the average atomic mass is arrived. $\endgroup$ – Mathivanan Palraj Aug 9 '16 at 1:11
  • $\begingroup$ Average atomic mass would mean the weighted average of the masses of br-79 and br-81 en.m.wikipedia.org/wiki/Isotopes_of_bromine $\endgroup$ – DavePhD Aug 9 '16 at 1:14
  • $\begingroup$ You are right and I was working on that. $\endgroup$ – Mathivanan Palraj Aug 9 '16 at 1:24
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Yes - they call it the Theory of Relativity. E=mc^2

Take all the mass of the protons/neutrons lost in the nuclear reaction (you need to know your isotopes here). That mass is a tiny one, but then multiple that by the speed of light squared, and you have the energy created from the nuclear reaction.

Bang! Thats it! [A very big bang in fact...]

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