The mass number is the combination of neutrons and protons within the nucleus. Thus, this atom has a total of 19 protons and neutrons combined. The atomic number is only the number of protons. Thus the atom has 8 protons. In order to find the neutrons, one must simply subtract the number of protons and one is left with the number of neutrons. 19 - 8 = 11. 11 neutrons.
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