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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.