Which Element is it?

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Across
  1. 2. placed in group 7A, kills bacteria
  2. 5. radioactive elements, only three exist in nature
  3. 10. used in computer chips and solar cells, second most abundant element in Earth's crust after oxygen, placed in group 4A
  4. 11. placed in group 3A, combines with silicon oxides to form borosilicate glass
  5. 13. lightest member of group 2A, used to moderate neutrons in nuclear reactions
  6. 14. silvery metals with relatively high melting points
  7. 16. placed in group 1A, most reactive alkali metal
  8. 17. placed in group 6A, can be combined with mercury
  9. 18. placed in group 8A, used in light displays
Down
  1. 1. lightest noble gas, discovered first in the emission spectrum of the Sun
  2. 3. placed in group 6A, most abundant element in Earth's crust
  3. 4. placed in 4A, contains one of the most important elements on Earth
  4. 6. a colorless, odorless and relatively unreactive gas that is placed in group 5A, about 78% of Earth's atmosphere is made of this element
  5. 7. an abundant element, can be formed into almost any shape, placed in group 2A, alloys with this element with aluminum and zinc are much light than steel but equally strong
  6. 8. lightest alkali metal, found in water, soil and rocks
  7. 9. placed in group 5A, the use of fertilizers containing this element can be harmful to the environment
  8. 12. placed in group 7A, reacts with nearly all of the elements, a deadly gas but its compound can save lives
  9. 15. placed in group 3A, it can melt in your hand