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My book says that a proton weighs 1.0073u, a neutron weighs 1.0087u, and an electron weighs 0.00055u.

Now, why is the mass of chlorine-35 equal to 34.969? Are there not 17 protons, 18 neutrons, and 17 electrons? I calculated it and it sums to around 35.29. Where did I go wrong?

12345bird
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1 Answers1

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You need to account for the energy released when nucleons and electrons come together and form a Cl-35 atom. Its called the Binding Energy.

This sort of equation can help to explain :

(Rest Mass Energy of Individual nucleons,electrons*) - (Various Binding Energies) = (Rest Mass Energy of Natural)

*edit

Potter
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  • ok thanks! i get it now, since the book did not say anything about binding energy. – 12345bird Apr 20 '20 at 05:10
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    Also, I used the term mass-energy. You can interpret it as actual mass, according to the mass-energy equivalence ( the famous E=mc^2 ). – Potter Apr 20 '20 at 05:13
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    @12345bird As mass of subatomic particles is used to be expressed in equivalent units of energy, usually MeV. – Poutnik Apr 20 '20 at 05:26
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    should be: (Rest Mass Energy of Individual particles, neutons, protons and electrons) - (Various Binding Energies) = (Rest Mass Energy of Natural Nucleus) // The electron binding energies are small compared to the nuclear binding energies, but easily measurable today. – MaxW Apr 20 '20 at 06:08
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    Oh noted, I'm not really up to date on new technologies. I'll make the appropriate edit. Thanks! – Potter Apr 20 '20 at 07:17