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Across 1. By far, most of an atom's volume is composed of this. 3. The amount of time required for half the amount of radioactive susbstance to decay 6. Fractionally charged particles that protons and neutrons are made of. 8. Atomic particle that carries a negative charge 11. A variety of an element that has a different number of neutrons. 15. Occurs because the nucleus has too many protons 16. What Thompson's model of the atom came to be known as. 20. Discovered that electrons jump to higher orbital states when it gains energy 21. Viewed electrons as "clouds". 22. Thought all objects were composed of earth, air, fire, or water. 26. Means "indivisible". 27. A particle that acts like a glue to hold protons together. 28. Part of the atom made up of protons and neutrons. 29. The force that binds protons to one another. 30. Atomic particle that has no charge associted with it. 31. Developed a tube that showed that all elements carry an electrostatic charge.

Down 2. What Dalton based his idea of the atom on 4. On the periodic table, this indicates the number of protons an element has 5. An atom that carries a charge 7. Equal to the number of protons added to the number of neutrons. 9. What Rutherford "shot" at his target in his famous experiment. 10. Recent research suggests that all particles may be fundamentally made of these. 12. Discoverd that the atom wsa primarily empty space with a dense, positively charged center. 13. Occurs when the neutron to proton ratio is too great 14. The type and form of the metal used by rutherford in his famous experiment. 17. The father of modern chemistry 18. Occurs because the nucleus is at too high of an energy state. 19. The first known person to write about the atom. 23. Discovered the electron 24. Discovered the neutron. 25. Atomic particle that carries a postive charge.