The Periodic Table

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Across
  1. 7. Variants of an element with different neutron counts
  2. 8. GASES Elements in Group 18 known for their chemical inertness
  3. 9. Neutral particles found in an atom’s nucleus
  4. 11. Positively charged particles in an atom’s nucleus
  5. 13. A highly reactive alkali metal essential for nerve impulses
  6. 17. The lightest and most abundant element in the universe
  7. 18. A noble gas used in some fluorescent lamps
  8. 19. A strong, lightweight transition metal used in alloys
  9. 20. METALS Highly reactive metals in Group 1 of the periodic table
  10. 21. A metalloid fundamental to computer chips and semiconductors
  11. 22. MASS Weighted average mass of an element’s isotopes
  12. 24. A diatomic gas making up about 78 percent of Earth’s air
Down
  1. 1. Pure substances made of only one type of atom
  2. 2. Horizontal rows of elements in the periodic table
  3. 3. Negatively charged particles orbiting the nucleus
  4. 4. A reactive metal crucial for chlorophyll in plants
  5. 5. A hard metalloid in Group 13 used in borax and fiberglass
  6. 6. A red-brown transition metal with high electrical conductivity
  7. 10. Another term for a set of elements with similar properties
  8. 12. A diatomic gas vital for respiration, about 21 percent of air
  9. 14. METALS Elements in Groups 3–12 with partially filled d orbitals
  10. 15. A noble gas known for its bright glow in signage
  11. 16. Vertical columns of elements in the periodic table
  12. 17. A noble gas used in balloons, the second element
  13. 23. The basis of organic compounds, forms diamond and graphite