The term symbol of a molecule with electronic configuration $\left(1 \sigma_{g}\right)^{2}\left(1 \sigma_{u}\right)^{2}\left(2 \sigma_{g}\right)^{2}\left(2 \sigma_{u}\right)^{2}\left(2 \pi_{u}\right)^{1}\left(2 \pi_{u}\right)^{1}$ is:
(a)${ }^{1} \Sigma_{\mathrm{g}}^{+}$
(b)${ }^{3} \Sigma_{\mathrm{g}}^{-}$
(c)${ }^{1} \Sigma_{\mathrm{g}}^{-}$
(d)${ }^{3} \Sigma_{\mathrm{g}}^{+}$
Answer
Answer: B ✓ checked by 4AB · confidence high
Explanation
Only (2πu)¹(2πu)¹ is open-shell. Λ = +1 − 1 = 0 gives Σ, S = 1 gives a triplet, u × u = g, and the σv reflection gives (−). Term: ³Σg⁻ (b).