Definately not a revision crossword for school subjects

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Across
  1. 4. 1st step of the scientific experiment process
  2. 6. twice a radius
  3. 8. Combination of atoms
Down
  1. 1. 9,5 Billion km
  2. 2. H2O
  3. 3. half of the diameter
  4. 5. 3.1415826
  5. 7. This shape has 3 sides
  6. 9. The 82nd element of the periodic table