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Is the actual atomic mass greater than the mass number?

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The atomic number is equal to the number of protons in the nucleus, whereas the atomic mass depends upon the sum of the mass of the protons and the neutrons (there are also electrons but they have much less mass than protons or neutrons). read more

Mass number is the number of protons plus the number of neutrons. This assumes that the mass of a proton or neutron is 1 Da (it’s actually 1.007276466879 Da and 1.00866491588 Da, respectively) and that electrons are massless. read more

The atomic mass is not equal to the atomic mass number because of isotopes. Isotopes are “siblings” of atoms. An atom is defined by the number of protons it has. While the majority of atoms have the same number of neutrons and protons, isotopes have a different number of neutrons than protons. read more

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