Chemistry Names (Difficult!!)

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Across
  1. 2. Mo
  2. 5. Na
  3. 7. SF6
  4. 9. Density close to gold (W)
  5. 11. Au
  6. 12. CO
  7. 13. CH4
  8. 17. N2O3
  9. 18. means five
Down
  1. 1. CO2
  2. 3. I3
  3. 4. Element in organic compounds
  4. 6. Pb
  5. 8. Br2
  6. 10. U
  7. 11. Metal melts at 88F
  8. 14. means three
  9. 15. Used in lights
  10. 16. Ag
  11. 19. means four