How is the mass number calculated for an element? a. the difference between the mass and the protons b. there is no calculation. c. the product of neutrons and protons d. the sum of protons and neutrons

The best answer is D.

The mass number of an element is the sum of the number of protons and the number of neutrons. Protons and neutrons both weigh about one atomic mass unit or amu.

The nucleus of an atom is made up of protons and neutrons. Both these two types of particles are almost the same size.  Almost all the mass of an atom (99%) is in the nucleus. Atomic mass therefore is really a measure of the size of the nucleus.

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