The molecule is said to be stable when

As per M.O theory, if  ${\text{N}}_{\text{b}} >{\text{N}}_{\text{a}}$then the molecule or ion is said to be high stable.  Here ${\text{N}}_{\text{b}}$represents the number of electrons present in bonding orbital where as  ${\text{N}}_{\text{a}}$ represents the number of electrons present in anti bonding orbital.