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.
In the 49-year jubilee cycle, both the first year (the jubilee year) and the last year (the 49th year) are important. At the time of Moses with the children of Israel, several key events occurred in the last of the forty years in the wilderness, which was the 49th year of the jubilee cycle.