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Across 5. one twelfth of the mass of a carbon-12 atom 7. weighted average mass of the atoms in a naturally occuring sample of the element 8. have the same number of protons but different number of neutrons 9. is the number of protons int the nucleus of an atom of a element 10. central core fo an atom 12. glowing beam 14. negatively charged subatomic particles 15. vertical column on the periodic table

Down 1. famous scientist's theories from his hypotheses 2. horizontal row on the periodic table 3. arrangement of elements 4. subatomic particles with no charge but witha mass nearly equal to that of a proton 6. the total number of protons and neutrons in an atom 11. smallest particle of an element 13. positively charged subatomic particles