4 . ▁A bove ▁this ▁number ▁a ▁struct ural ▁change ▁takes ▁place ▁with ▁an ▁ icos ah ed ral ▁arrangement ▁with ▁Cu + Ar 5 5 ▁and ▁Cu + Ar 1 4 6 ▁having ▁more ▁stability . ▁ ▁With ▁a ▁str ont ium ▁ion ▁Sr + ▁from ▁two ▁to ▁eight ▁arg on ▁atoms ▁can ▁form ▁clusters . ▁Sr + Ar 2 ▁has ▁a ▁triangle ▁shape ▁with ▁C 2 v ▁symmetry . ▁Sr + Ar 3 ▁has ▁a ▁trig onal ▁py ram id ▁shape ▁with ▁C 3 v ▁symmetry . ▁Sr + Ar 4 ▁has ▁two ▁trig onal ▁py ram ids ▁sharing ▁a ▁face ▁and ▁str ont ium ▁at ▁the ▁common ▁a pe x . ▁It ▁has ▁a ▁C 2 v ▁symmetry . ▁Sr + Ar 6 ▁has ▁a ▁pent ag onal ▁py ram id ▁of ▁arg on ▁atoms ▁with ▁the ▁str ont ium ▁atom ▁below ▁the ▁base . ▁ ▁N io b ium ▁t etra arg ide , ▁N b + Ar 4 ▁probably ▁has ▁the ▁arg on ▁atoms ▁arranged ▁in ▁a ▁square ▁around ▁the ▁n io b ium . ▁Similarly ▁for ▁van ad ium ▁t etra arg ide , ▁V + Ar 4 . ▁The ▁hex a arg ides , ▁Co + Ar 6 ▁and ▁Rh + Ar 6 ▁likely ▁have ▁oct ah ed ral ▁arg on ▁arrangement . ▁Ind ium ▁mon ocation ▁forms ▁clusters ▁with ▁multiple ▁arg on , ▁with ▁magic ▁numbers ▁at ▁ 1 2 , ▁ 1 8 , ▁ 2 2 , ▁ 2 5 , ▁ 2