Why does the atomic mass of an element differ from the atomic number? A. Atomic number consists of only the number of neutrons. Atomic mass also includes the number of protons. B. Atomic number consists of only the number of protons. Atomic mass also includes the number of neutrons. C. Atomic number consists of only the number of protons. Atomic mass also includes the number of electrons. D. Atomic number consists of only the number of electrons. Atomic mass also includes the number of protons.