Molecular Orbital Theory V: Practice with Sigma and Pi MO's

[gervais010]

how can you determine where the sigma and pi orbitals goes firsts with any atom? We know B2, C2, and N2, the pi-2p orbitals goes first followed by sigma-2p orbital. For the O2, F2, and Ne2 the sigma-2p orbital goes first followed by pi-2p orbitals. Please explain. Thank you

[holumyde1]

funny my professor always says goodluck when students leave the class due to his sucky teaching.

[arthurthefirst]

Same goes for the third example.

[arthurthefirst]

Yeah, I think he made a mistake on the two examples. C 2+ should be isoelectric with Beryllium, not Lithium, so it should be 4 valence electrons, not three. Also with Nitrogen 2-, it should be isoelectric with Fluorine, not Oxygen, therefore there should be 7 valence electrons, not 6.

[ChemAssistBeta]

The C2^2+ ion has two fewer electrons than a C2 molecule.

Each of the two C+ ions that are linearly combined to form the C2^2+ ion has one fewer electron than a C atom.

Combining two species that have one fewer electron than a C atom results in a C2 ion with two fewer electrons than a C2 molecule.

Is that a good answer?

[killbillov]

i'm so confused...when ur doing the c2^ 2+ example you said only one fewer electron?? why not 2 fewer? since it's 2+ you would loose 2 not just 1?

[jancoman]

@killbillov you take one from each side

[LiLNaRuT0Jr]

super helpful. thank you very much

[fashiongrl2010]

omg huge quiz today couldnt make it to SI (supplementary instruction) you saved my life! i thot it was atomic number you count but it seems to be valence.

[inneralpha]

Why does the energy between pie and sigma flip?

[linuxexperi]

@inneralpha actually as per energy consideration (Z=5--7) the MO orbitals are filled as S2s S2s* S2px PI2py PI2pz PI2py* PI2pz* S2px*, energy of 2s and 2px AO are fairly close, due small difference, interaction b/w these become large, results in loss of energy by S2s, S2s* which become more stable at the cost of S2px which get destabilized.