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This question already has an answer here:

1u is defined as per carbon 12 so 1 atom of c have 6 neutrons and 6 protons both having mass more than 1u (Google)so total mass of carbon 12 is more then 12u (also there would be some mass of electrons) but it should be exactly 12u as u is defined such.So what should i interpret.I am confused how this is possible

I am just a school student just entered chemistry

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marked as duplicate by andselisk, Loong Apr 1 at 7:34

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  • $\begingroup$ The relevant term is "mass defect". Very simply, some mass/energy is used to hold the charged particles together. $\endgroup$ – TAR86 Apr 1 at 7:06
  • $\begingroup$ @TAR86 Or, to put it another way, the mass of a nucleus does not, in general, equal the sum of the masses of its components because of nuclear binding energy. $\endgroup$ – matt_black Apr 1 at 13:36