Atoms & stuff

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Across
  1. 2. the last ring of electrons
  2. 4. that holds the proton and neutron
  3. 5. neutral in the nucleus
  4. 7. positive in the nucleus
  5. 8. building block of the universe
  6. 9. 17 on the periodic table
Down
  1. 1. 74 on the periodic table
  2. 3. negative in the nucleus
  3. 6. 35 on the periodic table
  4. 8. mass the mass of an atom