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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?

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  • $\begingroup$ It did not go wrong. The average mass per nucleon decreases toward iron, and then increases toward uranium. Where do you think nuclear fusion and fission take their energy from ? Check masses of 4He and 16O, or Fe isotopes. $\endgroup$
    – Poutnik
    Commented Apr 20, 2020 at 5:02
  • $\begingroup$ hi! my book said that all protons are the same weight, so goes with neutrons and electrons. where did i go wrong when I simply added the weights? $\endgroup$
    – 12345bird
    Commented Apr 20, 2020 at 5:05
  • $\begingroup$ You have not counted mass loss due their bound energy. $\Delta E = \Delta m \cdot c^2$ $\endgroup$
    – Poutnik
    Commented Apr 20, 2020 at 5:07
  • $\begingroup$ oh cool! my book did not teach about that yet, so good to know. Thanks for helping me! :) $\endgroup$
    – 12345bird
    Commented Apr 20, 2020 at 5:08
  • $\begingroup$ Related question and answer. $\endgroup$
    – Nicolau Saker Neto
    Commented Apr 20, 2020 at 5:10

1 Answer 1

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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

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  • $\begingroup$ ok thanks! i get it now, since the book did not say anything about binding energy. $\endgroup$
    – 12345bird
    Commented Apr 20, 2020 at 5:10
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    $\begingroup$ 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 ). $\endgroup$
    – Potter
    Commented Apr 20, 2020 at 5:13
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    $\begingroup$ @12345bird As mass of subatomic particles is used to be expressed in equivalent units of energy, usually MeV. $\endgroup$
    – Poutnik
    Commented Apr 20, 2020 at 5:26
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    $\begingroup$ 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. $\endgroup$
    – MaxW
    Commented Apr 20, 2020 at 6:08
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    $\begingroup$ Oh noted, I'm not really up to date on new technologies. I'll make the appropriate edit. Thanks! $\endgroup$
    – Potter
    Commented Apr 20, 2020 at 7:17

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