What is C-12

C-12 is  an isotope of Carbon and isotope are atoms  with the same number of protons, but differing numbers of neutrons.

SO Carbon isotope having 6 protons, 6 neutrons and 6 electrons and atomic mass 12u is referred to as C-12. 

carbon-12 is chosen as a standard for assigning the atomic masses. So atomic masses of all the elements are assigned with respect to carbon-12. 

1 atomic mass unit= 1/12 th of the mass of one carbon-12 atom.

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