Crossword puzzle of periodic table

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Across
  1. 1. Table of elements
  2. 9. The electrons in the outermost shell of an atom
  3. 10. Neutral atoms
  4. 13. different forms of a element, different masses
  5. 17. First period on the Periodic table
  6. 18. 17th period on the Periodic tabe
  7. 20. amount of protons in an element
Down
  1. 2. average mass of an element
  2. 3. negative atoms
  3. 4. what is on the periodic table
  4. 5. 3-12's period on the periodic table
  5. 6. Families of periodic table, goes vertical
  6. 7. The first element on the Periodic table
  7. 8. What the electrons and proton are in in an element
  8. 11. the horizontal groups of elements on the periodic table
  9. 12. the different levels of an atom
  10. 14. positive atoms
  11. 15. the amount of mass in an element
  12. 16. The tenth period on the periodic table
  13. 19. nickname for a element