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Across
  1. 3. A massive, luminous sphere of plasma held together by its own gravity, emitting light and heat.
  2. 5. A chemical element with the symbol He and atomic number 2; a colorless, odorless, and non-toxic gas.
  3. 8. All of space and everything in it, including planets, stars, galaxies, and all other forms of matter and energy.
  4. 9. A subatomic particle with no electrical charge found in the nucleus of an atom.
  5. 11. System A system comprising the Sun and the celestial bodies that orbit it, including planets, moons, asteroids, and comets.
  6. 12. The basic unit of a chemical element, consisting of a nucleus containing protons and neutrons, surrounded by electrons in shells.
  7. 13. The rapid expansion of matter from a state of extremely high density and temperature that marked the origin of the universe.
  8. 15. The process of combining two or more atomic nuclei to form a heavier nucleus, releasing a large amount of energy.
  9. 17. A tabular arrangement of the chemical elements, organized by their atomic number, electron configuration, and recurring chemical properties.
  10. 18. Groups of atoms held together by chemical bonds; the smallest unit of a chemical compound.
  11. 19. A pure substance made of only one type of atom that cannot be broken down into simpler substances by chemical means.
Down
  1. 1. The chemical element with the symbol H and atomic number 1; the lightest and most abundant element in the universe.
  2. 2. A measure of the amount of matter in an object; often measured in kilograms or grams.
  3. 4. Substance A material that has a constant composition and properties; can be either an element or a compound.
  4. 6. A subatomic particle with a negative charge that orbits the nucleus of an atom.
  5. 7. The star at the center of our solar system, around which Earth and other planets orbit.
  6. 10. The property of certain elements to spontaneously emit radiation as a result of nuclear instability.
  7. 14. The total number of protons and neutrons in an atom's nucleus.
  8. 16. A substance formed when two or more chemical elements are chemically bonded together.
  9. 17. A subatomic particle with a positive charge found in the nucleus of an atom.