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Across 2. credited with the discovery of the neutron 4. negatively charged subatomic particles 6. the simplest substances which cannot be broken down using chemical methods 7. houses protons and neutrons 8. Greek philosopher credited with the earliest idea of atoms existing 10. credited with the planetary model of the atoms that shows electrons orbiting the nucleus 14. the number of protons and neutrons (2 Words) 16. credited with the development of an early atomic theory to explain the behavior of atoms 17. credited with the discovery of the electron and the plum pudding model of the atom 18. any atom or group of atoms with a negative charge 19. subatomic particles with no charge 20. the horizontal rows of a periodic table

Down 1. Used the "gold foil" experiment to prove the existence of the nucleus 3. indicates the periodic element as well as the number of protons (2 Words) 5. the vertical columns on the periodic table 9. subatomic particle with a positive charge 11. form when atoms or ions join together to form new substances 12. atoms of the same element that have a different number of neutrons. 13. any atom or group of atoms with a positive charge 15. the number of electrons need to fill the outermost level of an atom 18. smallest particle of an element that retains the properties of that element