[lakealer96]

Hi!
Sorry for my English, i'm not English. I really need your help to claire all my doubts... ..
My question is : in MO theory two orbitals can combine to form two molecular orbitals, the bonding one (constructive interference) and the antibonding one (destructive interference). But, considering the two electrons in the 1s orbitals have always a positive value of width, how can they have a destructive interference?

Then, I have another doubt. If the antibonding orbital requires the interference of two electrons (which are waves), how can an antibonding orbital containing a single electron esiste (for example in O2)? What does this electron interfere with?

Thanks, this forum is great!

[Irlanur]

Considering the way orbitals are taught in many chemistry classes, your questions are far from surprising. In what kind of class did you learn about it?

While the "constructive/destructive" interference is a very visual concept, I think it's confusing many students (e.g. you). in the LCAO (linear combination of atomic orbitals) approach, we build molecular orbitals as linear combinations of atomic orbitals. In the easiest case of H2, we have two 1s orbitals: 1s₁ and 1s₂. you can now form different linear combination of these. the most relevant ones are   
1s₁ + 1s₂ and 1s₁ - 1s₂ (neglecting normalization). The first one would be the "constructive", the other one "destructive". Note that the electron density is the square of the wavefunction and that orbitals are in general complex-valued.

[magician4]

let me try to give you a picture ( and I know: all those analogies are defective to a certain extend) :

consider two atomic nuclei, let's say of hydrogen, i.e. protons, at a given small distance

what "options" do they offer to electrons coming along?

well, being positively charged (and both of them at that) , of course they'd welcome something with a negative charge coming along: electrostatics would favour such an arrangement

furthermore, the repulsion of those two nuclei would be diminished, if there was something negatively charged "right in the middle" , i.e. shared by both of them  net gain of stability for the system
(as you already know, "right in the middle" is not what really will happen: a different type of electron density distribution is what the outcome will be. nevertheless, the "sharing" part is ok, and gaining stability (hence: releasing energy) also will be what we'll see)

so: we expect H₂⁺ to be more stable than H₂²⁺, and that in fact is what we'll see in reality

however: let's imagine the electrons being bullet-shaped (i.e point type objects) as long as they're far away from those nuclei.
now, there's a rule for electrons if they wish to belong to those two nuclei, let's call it "hydrogen's rule".
the rule is: if you want to be with us, you'll have to disguise - no bullets allowed here. we only accept properly dressed members, meaning: waves.
furthermore, we don't allow more than two members to be dressed the same wave-type. that's the rule - take it or leave it.
... and a catalogue of possible wave types pls. find attached. some are more comfortable than others: we know ( that's why there's always a run for the most comfortable types still available).
However,  it's "first come, fist serve": completely "occupied" wave - types can't be chosen again, but you can chose freely from all those types not completely occupied yet.
... and we recommend to opt in for those that are most comfortable for all of us.

you find it disturbing that some of those options seem do be derived from something that looks like an interference pattern ? don't mind: that's just the two of us, me and the other nucleus, that's "our" little internal arrangement, that's what we have in store, if you wish to say so.

be welcomed and take it, if you please, and don't bother with the origin of our nutty li'll arrangement too much...





hope this helps?

regards

Ingo