periodic table

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Across
  1. 3. the mass of an atom of chemical element
  2. 5. a solid material that is typically hard
  3. 12. something that stands for or represents something else
  4. 14. subatomic particle with a zero charge
  5. 15. an element or substance that is not a metal
  6. 16. elements that form a salt when it reacts to metal
  7. 17. a simple way of writing down the locations of all of the electrons
  8. 18. horizontal lines on the periodic table
  9. 21. subatomic particle with a negative charge
Down
  1. 1. an element whose properties are in between metals and non-metals
  2. 2. basic unit of a chemical element
  3. 4. elements that form
  4. 6. subatomic particle with a positive charge
  5. 7. clear model of an atom
  6. 8. compounds
  7. 9. the number of protons in the nucleus
  8. 10. electrons in the outer shell of an atom
  9. 11. elements used to make steel and iron
  10. 13. a table of chemical elements
  11. 19. a part or aspect of something abstract
  12. 20. vertical columns on the periodic table