Periodic puzzle

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Across
  1. 3. heavy metal, once found in petrol
  2. 6. most reactive alkali metal
  3. 8. noble gas discovered through Curie's radioactive studies
  4. 9. element with electronic configuration 2.8.3
  5. 12. group 2 metal that burns with bright white light
  6. 13. soft, silvery metal that melts in your hand
  7. 14. non-metal present in graphite
  8. 16. non-flammable gas used in aerostatic zeppelins due to its low density
  9. 17. alloying partner of copper in brass
Down
  1. 1. gas produced when mixing an alkali metal with water
  2. 2. element with atomic mass 42
  3. 4. group 1 metal that produces a pink flame when mixed with water
  4. 5. halogen in period 6
  5. 7. transition metal that gives stainless steel its shine
  6. 10. element with 5 energy shells and 6 outer electrons
  7. 11. semi-metal in period 3 of the periodic table,widely used as a semiconductor in electronics
  8. 15. element with full outer shells in period 5