The mass number plays an important role for elements and their isotopes. Mass number comes from the addition of protons and neutrons (their weight). Isotopes are the elements, but with a different number of neutrons. So in turn, by subtracting the number of protons (atomic number) from the mass, you can find the number of neutrons.
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Protons and Neutrons, the electrons circle around the nucleus.
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Answer: a branch of chemistry concerned with the rate of chemical reactions, the factors that influence these rates, and the use of rate studies to clarify reaction mechanisms