Figure it out

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Across
  1. 2. it's just fire
  2. 5. is the symbol of TH on the periodic table
  3. 8. the most poisonous element
  4. 9. it's something a blant breathes in
  5. 11. very precious and are worth more than gold
  6. 13. this element group consists of fluorine
  7. 15. second lightest element
Down
  1. 1. poisonous and the heaviest on the periodic table
  2. 3. All in this group are radioactive and release energy upon radioactive decay
  3. 4. symbole of B with the atomic number 5
  4. 6. most reactive chemical element
  5. 7. this has the atomic number four with the symbole of BE
  6. 10. without it, we would die
  7. 12. 69th element on the periodic table
  8. 14. used in electric signs and fluorescent lamps