Question

Give the electron configuration and the term symbol corresponding to the diatomic molecule (CN). If more than one electronic state can arise for this particular electron configuration, rank the electronic states in order from lowest to highest energy. You do not have to assign +/- symmetry for any of the Σ electronic states you find.

I know the answer is Λ = 0, S = 1/2 and so 2Σ for CN, but I don't understand why Λ is 0 and not 1