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Across 2. in physical science, an atom's central region, which is made up of protons and neutrons. 4. an atom that has the same number of protons as other atoms of the same element do but that has a different number of neutrons. 6. the number of protons in the nucleus of an atom. 8. the sum of the numbers of protons and neutrons in the nucleus of an atom. 9. the smallest unit of an element that maintains the properties of that element. 10. a subatomic particle that has no charge and that is located in the nucleus of an atom. 11. a region around the nucleus of an atom where electrons are likely to be found.

Down 1. a unit of mass that describes the mass of an atom or molecule. 3. a subatomic particle that has a negative charge. 5. the mass of an atom expressed in atomic mass units. 7. a subatomic particle that has a positive charge and that is located in the nucleus of an atom.