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Across 2. subatomic particle with a positive charge 5. houses protons and neutrons 6. any atom or group of atoms with a positive charge 8. atoms of the same element that have a different number of neutrons. 9. the number of electrons need to fill the outermost level of an atom 15. credited with the discovery of the electron and the plum pudding model of the atom 16. credited with the discovery of the neutron 17. the horizontal rows of a periodic table 18. the vertical columns on the periodic table 20. indicates the periodic element as well as the number of protons (2 Words) 21. the number of protons and neutrons (2 Words)

Down 1. any atom or group of atoms with a negative charge 3. subatomic particles with no charge 4. credited with the development of an early atomic theory to explain the behavior of atoms 7. form when atoms or ions join together to form new substances 10. the simplest substances which cannot be broken down using chemical methods 11. Used the "gold foil" experiment to prove the existence of the nucleus 12. credited with the planetary model of the atoms that shows electrons orbiting the nucleus 13. Greek philosopher credited with the earliest idea of atoms existing 14. negatively charged subatomic particles 19. smallest particle of an element that retains the properties of that element