The mass (weight) of an atom of an element-Carbon equals 12.
The total mass of an atom, it is equal to the sum of the isotope’s number of protons and neutrons (in the atom’s nucleus). The atomic weights of the earth’s elements are based on the assignment of exactly 12.000 as the atomic weight of the carbon- 12 isotope (variation of atom). The atomic (weight) theory was established as a framework in 1869 by Meyer and Mendeleev, but standard precise values were not adopted internationally until an “international commission on atomic weights” was formed in 1 899 in response to an initiative by the German Chemical Society. An element’s atomic weight does not come out to a whole number (with the exception of carbon) because of the existence of isotopes which differ slightly with respect to the number of neutrons each contains.
The weight of an atom of an element compared with that of 1/12 the weight of carbon-12.
The number of protons and neutrons present in an atom is commonly referred to as the atomic mass or mass number