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.
Ode on a Grecian Urn. Ode on a Grecian Urn is a poem written by the English Romantic poet John Keats in May 1819 and published anonymously in the January 1820, Number 15, issue of the magazine Annals of the Fine Arts (see 1820 in poetry).