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Atoms contain three subatomic particles: protons, neutrons, and electrons. Protons have a positive charge and are found in the nucleus. Neutrons have no charge and are also in the nucleus. Electrons have a negative charge and move in shells around the nucleus. The atomic number equals the number of protons. The mass number equals the number of protons plus neutrons. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. The periodic table i