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I was reading Silberberg's principles of general chemistry when I noticed that it says that the mass of carbon12 is exactly 12 amu. But when I calculated the total mass of its components (6 protons : 1.00727 amu each , 6 neutrons : 1.00866 amu each and 6 electrons : 0.000548 amu each) it was obviously greater than 12. Why is that?

My explanation was that some of the mass is converted to energy but I don't know whether it was true or not.

And I'm sorry if my English is not good.

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marked as duplicate by DavePhD, jerepierre, bon, Todd Minehardt, ringo Oct 27 '16 at 23:46

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    $\begingroup$ That's right: when you put together 6 protons, 6 neutrons, and 6 (not 12, mind you) electrons, some of the mass will be lost as energy. Also, your English is fine. Also, welcome to Chem. SE. $\endgroup$ – Ivan Neretin Oct 27 '16 at 7:39
  • $\begingroup$ Sorry for the mistake (12 electrons) and thanks for your answer Ivan Neretin. $\endgroup$ – Ali Oct 27 '16 at 7:45

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