1. atomic number 91 a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead
  2. atomic number 95 a radioactive transuranic metallic element
  3. atomic number 11 a silvery soft waxy metallic element of the alkali metal group; occurs abundantly in natural compounds (especially in salt water); burns with a yellow flame and reacts violently in water; occurs in sea water and in the mineral halite (rock salt)
  4. atomic number 90 a soft silvery-white tetravalent radioactive metallic element; isotope 232 is used as a power source in nuclear reactors; occurs in thorite and in monazite sands
  5. atomic number 96 a radioactive transuranic metallic element
  6. atomic number 94 a solid silvery grey radioactive transuranic element whose atoms can be split when bombarded with neutrons; found in minute quantities in uranium ores but is usually synthesized in nuclear reactors; 13 isotopes are known with the most important being plutonium 239
  7. atomic number 99 a radioactive transuranic element produced by bombarding plutonium with neutrons
  8. atomic number 21 a white trivalent metallic element
  9. atomic number 41 a soft grey ductile metallic element used in alloys
  10. atomic number 61 a soft silvery metallic element of the rare earth group having no stable isotope; was discovered in radioactive form as a fission product of uranium
  11. atomic number 93 a radioactive transuranic metallic element
  12. atomic number 97 a radioactive transuranic element
  13. atomic number 19 a light soft silver-white metallic element of the alkali metal group; oxidizes rapidly in air and reacts violently with water; is abundant in nature in combined forms occurring in sea water and in carnallite and kainite and sylvite
  14. atomic number 31 a rare silvery metallic element
  15. atomic number 51 a metallic element having four allotropic forms
  16. atomic number 71 a trivalent metallic element of the rare earth group
  17. atomic number 81 a soft grey malleable metallic element that resembles tin but discolors on exposure to air; it is highly toxic and is used in rodent and insect poisons; occurs in zinc blende and some iron ores
  18. atomic number 92 a heavy toxic silvery-white radioactive metallic element
  19. atomic number 98 a radioactive transuranic element
  20. atomic number 1 a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe