# Two questions about MO diagrams [duplicate]

1. In $$\ce{HF},$$ the $$\mathrm{1s}$$ orbital of $$\ce{H}$$ matches the $$\mathrm{2p}$$ orbital of $$\ce{F}.$$ However, in $$\ce{LiF},$$ the $$\mathrm{2s}$$ orbital matches the $$\mathrm{2p}$$ orbital. I knew that the atom with the higher electronegativity goes lower, but is there a way to determine which orbital matches which?

2. In the MO diagram, the σ orbital goes under the π orbital in $$\ce{O2},$$ $$\ce{F2},$$ $$\ce{Ne2}.$$ However, it goes over the π orbital in $$\ce{B2},$$ $$\ce{C2},$$ $$\ce{N2}.$$ Does this difference happen in heteronuclear diatomics? If it does, how can I determine this difference in heteronclear diatomics?

• re. q2: I quote permeakra's comment on their answer chemistry.stackexchange.com/questions/22114 "It occurs in ALL molecules, the question is to what degree. Unfortunately, there is no way to predict if it will change relative position of orbitals in any particular case without some complex calculation. If relative position of orbitals is important, use some free package (say, us-gamess, free for personal use) to calculate orbitals and view them in some free viewer, say, avogadro. Fortunately, it is quite easy for 2-atom molecules these days." – orthocresol Aug 21 '19 at 19:19