− 1 . ▁The ▁significant ▁change ▁in ▁the ▁spectrum ▁occurs ▁because ▁the ▁C U O ▁is ▁changed ▁from ▁a ▁sing let ▁state ▁( in ▁gas ▁phase ▁or ▁solid ▁ne on ) ▁to ▁a ▁triple t ▁state , ▁with ▁arg on ▁or ▁noble ▁gas ▁complex ing . ▁The ▁arg on – ur an ium ▁bond ▁length ▁is ▁ 3 . 1 6   Å . ▁This ▁is ▁shorter ▁than ▁the ▁sum ▁of ▁atomic ▁rad ii ▁of ▁U ▁and ▁Ar ▁of ▁ 3 . 2 5   Å , ▁but ▁consider ably ▁longer ▁than ▁a ▁normal ▁c oval ent ▁bond ▁to ▁u ran ium . ▁For ▁example , ▁U − Cl ▁in ▁U Cl 6 ▁is ▁ 2 . 4 9   Å . ▁When ▁x en on ▁is ▁included ▁in ▁the ▁solid ▁arg on ▁matrix ▁up ▁to ▁a ▁few ▁percent , ▁additional ▁van ▁der ▁Wa als ▁mole cules ▁are ▁formed : ▁C U O · Ar 3 X e , ▁C U O · Ar 2 X e 2 , ▁C U O · Ar X e 3 ▁and ▁C U O · X e 4 . ▁Similarly ▁k rypt on ▁can ▁substitute ▁for ▁arg on ▁in ▁C U O · Ar 3 K r , ▁C U O · Ar 2 K r 2 , ▁C U O · Ar K r 3 ▁and ▁C U O · K r 4 . ▁The ▁shape ▁of ▁these ▁mole cules ▁is ▁roughly ▁oct ah ed ral , ▁with ▁a ▁u ran ium ▁centre ▁and ▁with