Two atomic orbitals combine to form...

[Le Boudin]

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I. a bonding molecular orbital
II. an antibonding molecular orbital
III. new atomic orbitals

Kaplan says it's I, II ONLY.
I think it's I, II, III because hybrid orbitals are indeed formed from atomic orbitals being combined.

Their solution says "New atomic orbitals do not form".

WTF?

 

[Rabolisk]

Well.. The two theories are completely different. I and II have to do with molecular orbital theory, and hybrid orbitals have to do with valence bond theory, which is at best an approximation.

 

[Le Boudin]

Yes I agree the two theories are different, but this isn't asking about which theory.

It's still correct, right? I have read several sources on the internet which describe hybridization exactly as "combining atomic orbitals" to form "new atomic orbitals".

And I thought the MCAT was all about putting concepts together.

 

[attixx]

I wouldn't stress too much about it.. seems like a technicality. You understand the concepts of MOs and hybridization and that's the important part. TPR does refer to hybrids specifically as "new orbitals", as does a few other sources I've seen online. But I guess Kaplan's take on it is that it's just a combination of existing orbitals so there is no "new" orbital. Seems stupid to me.
The chances of you running into a discrepancy like that in the future are small.

 

[chemtopper]

MO orbitals are as bonding and anti bonding orbitals
This is correct
Hybridization gives new hybrid orbitals. There is no new atomic orbital term .

 

[Le Boudin]

^So I guess the distinction is between "atomic" orbitals and "hybrid" orbitals.

THanks for the replies guys.

 

[ilovemcat]

I. a bonding molecular orbital
II. an antibonding molecular orbital
III. new atomic orbitals

Kaplan says it's I, II ONLY.
I think it's I, II, III because hybrid orbitals are indeed formed from atomic orbitals being combined.

Their solution says "New atomic orbitals do not form".

WTF?

Maybe I'm wrong here, but a hybrid orbital is formed via the same atomic orbital (not two different ones). It does this by promoting one of it's s-orbital electrons. So Carbon for example has an electron configuration of: [He]2s2 2p2. When it bonds with another atom, like Hydrogen, it promotes one of it's 2s orbital electrons into one of the 2p orbitals so that each orbital - the 2s and 2p orbitals all have 1 unpaired electron. This allows Carbon to make 4 different bonds.

Molecular Orbitals is the combination of electrons from 2 atomic orbitals. The way these electrons can combine is in a bonding or anti-bonding fashion. For instance, a single p-orbital has 2 signs (+/-) with a node in between. If you combine a p-orbital with another p-orbital that matches in sign, you form a bonding mo. If those signs are opposing (+/- and -/+) then that produces an anti-bonding mo. Both though are considered "molecular orbitals" because you're always combing one atomic orbital with another atomic orbital (from another atom).

 

[Rabolisk]

Maybe I'm wrong here, but a hybrid orbital is formed via the same atomic orbital (not two different ones). It does this by promoting one of it's s-orbital electrons. So Carbon for example has an electron configuration of: [He]2s2 2p2. When it bonds with another atom, like Hydrogen, it promotes one of it's 2s orbital electrons into one of the 2p orbitals so that each orbital - the 2s and 2p orbitals all have 1 unpaired electron. This allows Carbon to make 4 different bonds.

4 sp3 hybrid orbitals are then formed from the s orbital and 3 p orbitals each with one electron, which is why one can say that 2 atomic orbitals can form new hybrid orbitals. It wouldn't be right to say that you have 1 s orbital and 3 p orbitals, because then, 1 of those electrons would be lower in energy. That would lead to one C-H bond in methane being different energetically from the other three C-H bonds, which is not true.