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The mass number is defined as the NUMBER of protons and neutrons. Electrons aside, why does the number of protons and neutrons equal the atomic mass rounded to the nearest whole number? That would mean, to me, that the number of protons and neutrons equals almost exactly to their relative mass! Why is that true?

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    $\begingroup$ Because proton weighs (well, almost exactly) 1 and neutron also weighs 1; what's unclear about that? $\endgroup$ – Ivan Neretin May 16 '18 at 6:22
  • $\begingroup$ How about the isotopes? $\endgroup$ – HolgerFiedler May 16 '18 at 12:11
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This is incorrect. The atomic mass you read off the periodic table is the weighted average of all isotopic masses. For the most part, the masses of the isotopes are just the sum of the protons and neutrons, but nuclear binding energy causes some deviation. Also, protons and neutrons don't have the same mass, so for larger atoms, that will also cause deviation from a simple sum.

The biggest issue like I said is that atomic mass is the weighted average. For example, the two most prevalent isotopes of bromine are Br-79 and Br-81, present in about the same relative abundance. However, the atomic mass is 79.9 because it is the average. That rounds to neither 79 or 81.

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  • $\begingroup$ What is incorrect specifically? The atomic mass rounded to the nearest whole number is defined as the mass number. And 79,9 does NOT round to 81. I'm unclear as to how you are explaining this. $\endgroup$ – suse May 17 '18 at 15:59
  • $\begingroup$ That's my point. The mass number is the sum of protons and neutrons. The atomic mass is the weighted average of the masses of individual elements weighted by abundance. The atomic mass of bromine is 79.9. Bromine atoms do not have a number of protons and neutrons totaling 80. $\endgroup$ – Zhe May 17 '18 at 17:02

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