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?
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.
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...]