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Author Topic: Molecular Orbital Diagram of O2.  (Read 3300 times)

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natalie

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Molecular Orbital Diagram of O2.
« on: December 18, 2011, 10:38:58 AM »

I thought I understood the molecular orbital theory but this practice question is making me confused.

1. For a diatomic molecule of oxygen (O2), we define the bond as lying along the z axis.
a) Draw the valence MO diagram for O2. Include labels for molecular orbitals.
----> For this, I drew the MO diagram but was wondering, since it says 'valence' MO diagram, do I draw from 2s and 2p and not draw 1s since the valence electrons are in 2s and 2p? Also, it says that the bond is lying along the z axis, does that mean the sigma bond is in the z axis and the pi bonds are in the x and y axis?

b) Which atomic orbitals combine to form sigma MOs in O2? Be specific
----> For this, do I write the hybridization (sp2) or do I write s and p make sigma or something else?

c) Which atomic orbitals combine to form pi MOs in O2? Be specific.
---->Again, I don't understand what it's asking for.

d) Write the orbital occupancy for O2.
---->I don't know what this means because I don't remember my professor talking about this and she said not to use the textbook because its wrong.

e) What is the net sigma bond for O2?
----> Would i just count all the electrons in sigma and sigma stars which would be 10?
f) What is the net pi bond for O2?
----> Again, would I count all the electrons in pi and pi star to give me 6?

g) What is the overall bond order of O2?
----> For this, do I just use BO=(#e-in bonding - #e-in antibonding)/2 to give me 2.

h) How many electrons must be added to O2 to reduce the bond order to zero?
---->For this one, would it be four because then it would be BO=(10-10)/2 because instead of six electrons in antibonding, I would have 10 electrons in antibonding?
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awkko808

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Re: Molecular Orbital Diagram of O2.
« Reply #1 on: December 18, 2011, 12:33:09 PM »

I thought I understood the molecular orbital theory but this practice question is making me confused.

1. For a diatomic molecule of oxygen (O2), we define the bond as lying along the z axis.
a) Draw the valence MO diagram for O2. Include labels for molecular orbitals.
----> For this, I drew the MO diagram but was wondering, since it says 'valence' MO diagram, do I draw from 2s and 2p and not draw 1s since the valence electrons are in 2s and 2p? Also, it says that the bond is lying along the z axis, does that mean the sigma bond is in the z axis and the pi bonds are in the x and y axis?

b) Which atomic orbitals combine to form sigma MOs in O2? Be specific
----> For this, do I write the hybridization (sp2) or do I write s and p make sigma or something else?

c) Which atomic orbitals combine to form pi MOs in O2? Be specific.
---->Again, I don't understand what it's asking for.

d) Write the orbital occupancy for O2.
---->I don't know what this means because I don't remember my professor talking about this and she said not to use the textbook because its wrong.

e) What is the net sigma bond for O2?
----> Would i just count all the electrons in sigma and sigma stars which would be 10?
f) What is the net pi bond for O2?
----> Again, would I count all the electrons in pi and pi star to give me 6?

g) What is the overall bond order of O2?
----> For this, do I just use BO=(#e-in bonding - #e-in antibonding)/2 to give me 2.

h) How many electrons must be added to O2 to reduce the bond order to zero?
---->For this one, would it be four because then it would be BO=(10-10)/2 because instead of six electrons in antibonding, I would have 10 electrons in antibonding?


1. For a diatomic molecule of oxygen (O2), we define the bond as lying along the z axis.
a) Draw the valence MO diagram for O2. Include labels for molecular orbitals.
----> You're right by excluding 1s, but if they're asking for valence orbitals (the question says valence MO) I learned that only the highest energy, unfilled set of orbitals of each atom are involved. In this case, only the 2p orbitals of oxygen would be considered valence orbitals and when they combine, the bonding and anti-bonding sigma-2p and pi-2p MOs would be considered valence MOs. This is confusing because we commonly refer to the term valence shells to describe electrons of the highest principal quantum number 'n' or the 'energy level' of the 'outermost' electrons. So to contrast, oxygen's valence shell IS in energy level 2, as in 2s2p, but its valence orbitals would only be 2p. However, this you should probably clarify with your professor or other students because I don't think this distinction is widely clarified.

As for the bond on the z-axis, I think this is conventional as this is what I've been seeing in textbooks. The sigma 2p MO always seems to lie on the z-axis. Thus the 2pz AOs would be involved in a sigma MO.

b) Which atomic orbitals combine to form sigma MOs in O2? Be specific
----> It seems like all they're asking for is the 2pz AOs? Since the 2px and 2py AOs form pi MOs. I don't think you had to include anything about hybridization here.


c) Which atomic orbitals combine to form pi MOs in O2? Be specific.
---->I'd assume 2px and 2py, assuming that the premise that the 2pz AOs form the sigma 2p MO (as answered in 1a and 1b) is correct.

d) Write the orbital occupancy for O2.
---->No idea but could it be asking for electron configuration?


As for e-h those all seem right or make sense. I hope this information from a fellow chem student helps!
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StibAMDG

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Re: Molecular Orbital Diagram of O2.
« Reply #2 on: November 19, 2012, 03:06:55 AM »

I know this thread is from a long time ago, but I thought I would clear things up for those who see this in the future.  The MO diagram for O2 can be found readily by just googling it.  A pretty nice explanation can be found at http://depts.washington.edu/chemcrs/bulkdisk/chem162A_sum05/handout_Lecture_02.pdf
But what it means, i.e. questions e-h, is what matters. 

e) For the net sigma bond, you would add all the electrons in σ orbitals and subtract the ones in σ* orbitals.  Thus, if you just look at the MOs coming from the 2p orbitals, you have 2 electrons in σ(2p) which results in a net sigma bond of 1 since it takes 2 electrons to make one bond. 

f) You also have 4 electrons in the π(2p) and 2 in the π*(2p), so 4 bonding minus 2 antibonding electrons, gives you a net of 1 pi bond. 

g-h) The rest of the questions you have right.  The bond order is 2 (1 σ, 1 π), and 4 more electrons would give a total bond order of 0, which is why the He2 molecule does not exist.
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