[Stanman]

An element, X, forms several compounds with oxygen.  Two of them are XO2 (18 valence electrons) and XO3 (24 valence electrons).  X is capable of expanding its octet.
 
Part A -
Element Identity
Possible options for the identity of X include: aluminum (Al), carbon (C), phosphorus (P), sulfur (S), and xenon (Xe).  Which element is X? I thought it was sulfur which was indicated as correct, I changed my answer for element X later because I realized that sulfur can't form several compounds with oxygen and that xenon could. Anyways regardless my professor makes statements that I don't agree with but want to make sure I'm not losing points for no reason. If someone could help me who has a really good understanding that would be most appreciated as I need every point I can get. Thanks

   
xenon (Xe)
   
carbon (C)
   
sulfur (S)
   
aluminum (Al)
   
phorphorus (P)
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Correct
Part B -
Lewis structures.
Draw the best Lewis structures for XO2 and XO3.
To add lone pairs, click the  button below (which will turn yellow when activated) before clicking on the molecule.
Draw the molecule by placing atoms on the grid and connecting them with bonds. Include lone pairs of electrons. For part B he had one picture with phosphorus bonded to 3 hydrogens and 1 unshared pair on the phosphorus. I don't know how that can be when element X is supposedly Sulfur and he has for the key one diagram of phosphorus.

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Incorrect; correct answer displayed
Part C -
Formal Charge
True or false:  X has the same formal charge in both XO2 and XO3.
In part C I got wrong originally because I changed my mind for Xe because once you submit answer you can't change it nor does it show correct answer, (assuming it's Sulfur) but at this point not sure because how he contradicts himself in part B. Sulfur has 2 structures with a formal charge of 0, (6-6=0). So I agree with true if it is Sulfur (later he admits its sulfur but no idea why he used phosphorus in part B)
   
True
   
False
   
Cannot be decided based on information given.
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Incorrect; correct answer displayed
Part D -
Bond angles
What are the bond angles around X in XO3?
The XO3 bond angles will be 120 degrees since it has a trigonal planar molecular geometry.
SubmitMy AnswersGive Up  This answer is correct for Sulfur, but to his phosphorus diagram, its tetrahedral (electron domain) and its molecular shape is trigonal pyramidal, meaning it would be 109.5?
Graded, see 'My Answers' for details
Part E -
True or false: XO2 and XO3 have the same arrangement to their electron domains (bonding and nonbonding combined). True is correct answer although I had Xe mindset at the time

True
   
False
   
Cannot be deteremined from information given.
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Correct
Part F
A fellow student claims that the bond strength between X and O in both compounds is the same.  Do you agree or disagree with this statement?  Explain your decision.
Yes  I agree, bond strength is the same due to both having single bonds. Both are polar because of the difference in electronegativitiy and neither is symmetrical. He agreed for part F mostly (only 75% credit), but not G. His answer for F: (Agree. In the best Lewis structure for each compound, S has double bonds to each oxygen, which means the bond strengths are the same.)
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Graded, see 'My Answers' for details
Part G
Another student claims that XO2 has a dipole moment while XO3 does not.  Do you agree or disagree with this statement?  Again, explain your decision.
I disagree, both have dipole moments because they have polar bonds. Kept it brief cause I was low on time. But still disagree.
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Graded, see 'My Answers' for details His answer ( In both compounds, the X-O bonds are polar. In XO3 the centers for both positive and negative charges are the same, and so, it has no dipole moment. XO2, however, has charge separation because of its bent shape. The negative pole would be around the oxygen atoms and the positive pole around the X.)


This is an online summer course and he doesn't teach (no lecture whatsoever) and you have to teach yourself. I just want to learn and not trying to sound like  I'm attacking the professor, but I really feel that he has graded wrong. Thanks

[Dan]

SO₂, SO₃, XeO₂ and XeO₃ all exist.

The Xe oxides are quite exotic - S is the more obvious answer, but both are valid. Unless X = Xe is an accepted answer, this was a badly designed question.

As for the rest of your post, it is confusing and has a lot of superfluous copy/paste - "Cannot be decided based on information given. SubmitMy AnswersGive Up Incorrect; correct answer displayed" etc. - that I am not interested in navigating. Maybe someone else will have the time, but I suggest you tidy it up (you will get more responses that way).

[mjc123]

The Xe oxides would have the wrong number of valence electrons.

The XO3 bond angles will be 120 degrees since it has a trigonal planar molecular geometry.

This answer is correct for Sulfur

Is it?

True or false: XO2 and XO3 have the same arrangement to their electron domains (bonding and nonbonding combined). True is correct answer

So can you see why SO₃ is not trigonal planar?

Part F: He is approximately correct, since the bonds are all double (not some double, some single etc.), but they are unlikely to be exactly the same in the two compounds. (From the heats of formation, I estimate bond energies of 537 and 474 kJ/mol in SO₂ and SO₃ respectively.)

Part G: Neither of you is completely right. They both have dipole moments, but simply having polar bonds is not sufficient for the molecule to have a dipole moment. The dipole moment is the vector sum of the bond dipoles (and the lone pairs), and this can be zero if the symmetry is right. If SO₃ was trigonal planar, as he seems to erroneously believe, it would have zero dipole moment despite having polar bonds. But it isn't.

[Dan]

So can you see why SO₃ is not trigonal planar?

I think SO₃ is trigonal planar - sulfur trioxide, not sulfite ion.

[mjc123]

Yes, you're right - oops! XeO₃ would be pyramidal.
So the teacher was right about the dipole moment then.

[Stanman]

Thanks very much for the feedback and your time. My bad on the messy copy and pastes. I will organize my information the next time if I ever need help.