An atomic physicist determines that an atom is composed of 8 positively charged particles and has a mass of 15 amu. Which is the best conclusion that can be drawn?

The positively charged particles in an atom are protons, so the atom has 8 protons. Its mass number is 15, which is the total number of protons and neutrons; 15 − 8 = 7 neutrons. Therefore, the best conclusion is that it has 7 neutrons and 8 protons.

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