Basic chemistry

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Across
  1. 2. Symbol for water
  2. 6. Center of an atom
  3. 10. Subatomic particle with no charge
  4. 11. Horizontal row in periodic table
  5. 16. The pH of a neutral substance
  6. 17. Negatively charged particle
  7. 19. Smallest particle of an element
  8. 20. Number of protons in an atom
  9. 21. Gas needed for combustion
Down
  1. 1. Mixture that looks uniform
  2. 3. Device used to measure mass
  3. 4. Tool used to heat substances
  4. 5. Table organizing elements
  5. 7. Vertical column in the periodic table
  6. 8. Process that forms new substances
  7. 9. Bond formed by sharing electrons
  8. 12. Abbreviation for sodium chloride
  9. 13. Simplest type of pure substance
  10. 14. Positively charged particle
  11. 15. Unit of temerature in science
  12. 18. Electron shell or ______ level
  13. 20. Opposite of base