I understand the bulk of MO theory , the formation of the bonding and anti bonding MOs etc. However, the topic about s-p mixing is quite puzzling to me. As a general rule of the thumb, I was taught that diatomic molecules up to N2 would exhibit s-p mixing, while the molecules past that threshold, O2 and above, would have insufficient amount of s-p mixing for the bonding MOs to be swapped. So my question is for heteronuclear molecules , eq CO , NO etc, how would I know whether s-p mixing occurs for that molecule? Ive found some sources online stating that molecules with <= 14 electrons would show s-p mixing, but this doesnt work for the NO molecule as it has 15 electrons and apparently exhibit s-p mixing