I really don't understand anti-bonding orbitals. Each molecule is bound together by shared electrons (from what I am assuming is bonding orbitals which are lower in energy than the two constituent atomic orbitals that make up the molecule). Where do antibonding orbitals come into play, and why do they even exist if molecules form and become more stabilized (hence don't occupy higher anti-bonding orbital states)? Also, does each n shell have bonding/antibonding orbitals or is it each "l" orbital (s, p, d, f) within each shell?
Bonding and antibonding orbitals exist because of the wave properties of distributing electrons in a bond between nuclei. Understanding why isn't really important, just know that mathematically the positional variability of electrons necessitates distributions between the nuclei (bonding) and everywhere but between the nuclei (antibonding). Considering those two possibilities, it's easy to see why they are so named.
Molecules form because the electrons occupying bonding orbitals outnumber those occupying antibonding orbitals. Half the difference is called the bond order, and this is equal to the number of bonds we draw between the atoms in a Lewis structure.
See below for my comments about VSEPR orbitals.
Having taken the MCAT several times, I believe knowledge in bonding/antibonding is important.
Also - if anybody can tell me how you know in the orbitals whether it is sigma or pi?
In which orbitals? VSEPR theory orbitals (s, p, d, etc.) are not considered in sigma and pi bonding because bonding and antibonding orbitals are constructs of MO theory, which is separate. VSEPR theory orbitals are atomic orbitals, used for describing the distribution of electrons in atoms not involved in bonds. Hybridization of VSEPR orbitals provides one solution, but MO theory provides another altogether.
If you want to know how to identify sigma and pi bonds, every single bond between atoms is a sigma bond. Every double bond contains that sigma plus one pi, and every triple contains the sigma plus two pi.
I'd also like to point out that VSEPR theory is explicitly listed in both the BS and PS topics lists, but MO theory is not. If you've seen MO theory questions on the MCAT that's one thing, but I wouldn't have expected them.
Draw a Molecular Orbital (MO) diagram so that you can see the e-'s in the bonding and anti bonding orbitals sigma for s orbitals and pie for p orbitals. They fill bonding sigma to antibonding sigma to bonding pie to anti bonding pie. So something like He2 would have two electrons in the bonding sigma s orbitals and two in the antibonding. It's important for the bond order so that you'll know the number of bonds between two atoms. Hope this helps.
This is only correct in the case of very low atomic number elements; see the video below to see how it differs.
Thanks. So are antibonding electrons the electrons that are not in the bond? So O2 would have 8 antibonding electrons and 4 bonding electrons from the double bond?
No. The problem with this thinking is that in reality the electrons aren't discretely isolated as lone pairs and such. All valence electrons are involved in the bonding in some way, but it's not as direct a correspondence as to say that the ones we draw as sitting in the p orbitals (the lone pair electrons) are the ones in antibonding orbitals; they're not.
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As a closing thought I have always found MO theory much more challenging than VSEPR theory and other treatments of electron behavior encountered in general and organic chemistry. I would wager very few, if any students that complete those courses at my university actual have a complete understanding of MO theory to the extent that it is presented in our gen chem book.
