Generally, the more empty the outer electron shell, the more reactive the element. Helium has two(2) electrons, and is thus full, all the other noble elements has eight(8) electrons in it's outer electron shell, so they are also full. *And for everyone's sake, krypton is not green, but colorless, and forms white crystals when solid.
(a) Jubilees was written between 153 B.C. and the year of Hyrcanus' breach with the Pharisees. (1) It was written during the pontificate of the Maccabean family, and not earlier than 155 B.C., when this office was assumed by Jonathan the Maccabee.
LibriVox recording of THE GILDED AGE, A TALE OF TODAY, by MARK TWAIN & CHARLES DUDLEY WARNER. Read by JOHN GREENMAN. Originally published in 1873, The Gilded Age: A Tale of Today is the only novel Twain co-wrote (C.D. Warner was a good friend and neighbor of the Clemens family in Hartford, and the collaboration sprang from their wive's challenge and encouragement).