Questions and Answers for 20A-1

Question:

Hello Professor. I was wondering if you could please tell me the average on the final and what the curve looked like? I would greatly appreciate it. Thank you.

Answer:

The average was 142/200, corresponding to a B-/C+. Hope you had a good holiday!

Question:

First of all professor, as banal as it may sound, i really want to commend you on the effort you put forth in this class to ensure that we learned as much as possible and that everything was fair. You're the only teacher i know who had an office everyday and that made every effort to help us learn the material. Secondly, I didn't put down any stamps on my final, but can you make sure you hold on to my final so that i can take a look at it when I get back from our winter intermission. thanks a bunch professor. i hope to have you in the future considering i am a biochem major. thank you and have a happy holidays.

Answer:

Thanks for your note. You can stop by any time and pick up your final. Hope you had a good holiday.

Question:

Hello, Professor Gelbart. I wanted to let you know that I enjoyed being in your class and you really made Chemistry more interesting for me. Also, I wanted to ask you when our final grades will be available?

Answer:

I'm glad you enjoyed the course. I did too! Your grades are available any time now; just stop by.

Question:

I was wondering if we didn't bring stamps to the final, can we pick up the final at the beginning of next quarter? thanks

Answer:

Yes!

Question:

When the tests are graded would it be possible to email me my test grade, and is possible my final grade. Thank you very very much. -James Scott Henderson 902860705 scotty@ucla.edu

Answer:

Sorry, but I'd rather not do it by email, even with your ID number.

Question:

If, and When, are you gonna post up the answers to the final, and are you gonna post up a grade distribution with it as well?

Answer:

No. But I can tell you that the average on the final was 142/200.

Question:

Personally I think that submiting humerous questions can enliven a class and makes it more interesting to read the VOH page. Quit sketching so much. If you think the question is stupid, SKIP IT! But I want to know the answer as well!

Answer:

How am I supposed to answer this?!

Question:

here's a thoutht: why are people posting #$%*@^! stupid questions at such a crucial time?

Answer:

Yes....

Question:

Professor Gelbart... My friends and I are curious...What country is your accent from?

Answer:

I was born and grew up here in the States, and my parents had no accent. Just me and my identical twin brother, and I don't know where we got it from!

Question:

i'm very lost on the review questions. could you explain how to do number one. i was also wondering if you will be around at all today before the final. i thought i understood the material on the 2nd midterm but now i'm confused on the review. on #6, is the equation for the particle in a box E=(h^2 n^2)/(8mL^2) and is n^2= to 4 or 3? please get back to me soon for i am really really stressing.

Answer:

Sorry I couldn't get back to you before the exam!

Question:

i'm very lost on the review questions. could you explain how to do number one. i was also wondering if you will be around at all today before the final. i thought i understood the material on the 2nd midterm but now i'm confused on the review. on #6, is the equation for the particle in a box E=(h^2 n^2)/(8mL^2) and is n^2= to 4 or 3? please get back to me soon for i am really really stressing.

Answer:

There wasn't time to get to your question before the exam -- sorry!

Question:

oops.. i'm a dumb. my question got answered. thanx anyways Prof. Gelbart.

Answer:

OK! And Happy New Year.

Question:

umm... on problem set #9, problem number 54. 2 of the isomers are the same thing, just rotations of each other. aren't they? for the CO[(NH3)4(CL2)2] atom. Also, on page 672 of the book it says that there are only 2 isomers, so i'm confused. If you could just clarify this, that would be great. thanks. =)

Answer:

Sorry I didn't respond sooner. I'd be glad to see you at one of my office hours starting next week.

Question:

In the extra problems handout, we had to draw a correlation diagram for NO-. My question is, which correlation diagram should we use for this (these are listed on pg. 588 in the book). Is there a reason why we should choose one over the other, or should we use both? I think I may have read somewhere that we should use both, meaning that we must draw 2 correlation diagrams for NO-, but I'm still unsure.

Answer:

If you are still interested, let's talk during one of my office hours next (this!) quarter.

Question:

how do you do the first question of the review sheet? also, EA= IE in magnitude right? (i think question 7 asks us to solve for EA and IE)

Answer:

Sorry I didn't respond sooner. If you are still interested, come by to one of my office hours starting next week. Happy New Year.

Question:

Thank you Professor Gilbart for explaining and teaching to me everything this quarter. I really enjoyed your class. I have one problem... I accidently thought that 2-33cent stamps would suffice for 77cents. But clearly, I am wrong. I was wondering if I can drop off another 33 cent stamp. Thank you. Pauline Chang

Answer:

Yes, of course: drop off a stamp any time during the week, at your convenience. Glad you enjoyed the course -- so did I!

Question:

hello. i was wondering, for the final on dimensional analysis should we memorize some of the dimensions such as the permittivity of vacuum and planks constant or will you provide them for us? thanks!

Answer:

If you think you need, during the exam, to know these dimensions, just ask!

Question:

you are a very funny man

Answer:

Is that a compliment?

Question:

Could anyone answer this please, where is the final tomorrow??

Answer:

Students whose last name begins with A through L should go to Moore 100; M-Z go to CS50.

Question:

Professor Gelbardt, I'm a little unsure about the definitions of excitation energies and excitation thresholds. My understanding is that excitation energies are the allowed energies of an electron (for example, 13.6 eV for the first excitation energy of Hydrogen), and that excitation thresholds are the differences between an allowed energy when n=2,3,4... and the allowed energy when n=1 (for example, 10.2 eV for the first excitation energy of hydrogen). Are these definitions correct?

Answer:

Actually, BOTH excitation energies AND excitation thresholds are the same thing, i.e., the first of both is E2-E1, and the second of both is E3-E1, and so on. Is that clear?

Question:

Prof. Gelbart, I wasn't able to make it to the review session of Friday and I was wondering if you told the class the constitution of the final. So would you be able to tell me what's going to be on the final if you told the class during the review. Moreover I saw that you were able to give out estimate of the grade with one of the student and would you be able to tell me where I stand right now so that I have an idea how the grade I have to get on the final. My student id # is 902847969. So would you be able to respond as soon as possible, that would help a lot. Thanks.

Answer:

I didn't tell the class at the review session any more than I did last Wednesday in lecture. The final will be emphasizing the sorts of things that I included in the review problem handout from last Wednesday. As for grades, I'm not clear what "estimate" you are referring to, but all you need to know now -- at this very late date -- is that the final is a big part of the overall grade; if you do well on it, you can't do badly in the course. But you know that already! Good luck!

Question:

I have another question Prof. Gelbart. Obviously, we need to be familiar with the equations that we've studied during the quarter. Are we going to be responsible for memorizing them?

Answer:

No, just as I have answered each time! If, during the final, you think you need to know some equation that isn't given on the back pages, just ask!

Question:

Prof. Gelbart- I have two questions. 1) In a problem like #11 on the review problems, do we need to write out the molecular configuration of the bonds? and 2) in the particle-in-a-box model, when an excited stated exists, how do you know whether it is the n(x), the n(y), or the n(z) that changes? (example 15.8 (b) p.550)

Answer:

First, about #11. Yes, you have to "write out" the configuration in the sense that I want you to be explicit about the nature of the orbitals involved -- e.g., sigma vs pi, and localized vs delocalized, and bonding vs antibonding, etc. -- AND their populations. Second, about the particle-in-a-box question. Indeed, each excited state is different, according to whether n(x) has changed, or n(y), etc., and one needs to specifiy which is involved.

Question:

I just wanted to know what I was getting in the class so far, before taking the final. I know it may be a little late, but I wanted to see if it would be better for me to either drop the class or take the final. It would be great if I can get the answer by early tomorrow because the deadline for dropping classes. Thank you. student id #202862171

Answer:

It's hard to say, because the final exam is so important. But obviously you are not doing well -- C- or D so far; and if you haven't been doing a good job with the homework, it's likely you won't do better on the final. So it's a tough decision that you will need to make. Good luck!

Question:

Professor Gelbart, I'd truly appreciate it if you would post the answer to problem set #8 and #9 so that we could use it as a review.

Answer:

They were submitted to voh yesterday, and should be "up" today, Thursday the 9th.

Question:

In the graph, r^2wavefunction^2 vs r^2 we can see the shielding 1s, 2s, and 2p. And you said that because of shielding Zeff is larger in 2s than 2p. From this I have a question on why this happens. Using your Zeff model I see that it is true, but I don't know if my reasoning is correct when I am using the graph to show that the Zeff 2s > zeff 2p. (1) Part of 2s is shielded by 1s so the "neg" charge of the shielded part of 2s is not felt by the nucleus, making the Zeff larger. And there is less shielding in 2P so there is a larger negative charge making the Zeff in 2p smaller. or (2) there is more shielding in 2p, which blocks the positive charge of the nucleus to effect the valence electrons in the 2p, so the Zeff is smaller in 2p than 2s (less shielding)

Answer:

It's because more (relative to 2p) of the 2s probability "sits inside" the 1s cloud that Zeff is bigger for 2s (than for 2p), hence lowering its energy below 2p.

Question:

What are the dimensions for a Coulomb? I thought a read somewhere that it is the same as a Joule, but I'm not sure?

Answer:

Coulombs are fundamental units in the SI system, like meters, kilograms and seconds. Joules are compound units, i.e., kg times meters*squared/seconds*squared. For dimensional arguments/analysis, it helps to realize that e(charge)*squared divided by the permittivity of vacuum has dimensions of energy times distance. Does that help?

Question:

Please during tomorrow's class explain what will be on the exam, especially material after Thanksgiving. Thank you

Answer:

OK!

Question:

Prof. Gelbart- In drawing the structures of octahedral complexes, like in problem 24 on p. 690, do mirror images count as isomers?

Answer:

Two distinct congigurations -- i.e., ones that can not be superimposed by an overall rotation -- are considered to be different ISOMERS. If, in addition, they are mirror images of one another, then they are called OPTICAL isomers.

Question:

Prof. Gelbart- on page 666, the book mentions the case involving ethylenediamine bonding to Co(3+) to form a coordination complex. How do six nitrogen donors bind to a single Co (3+) ion?

Answer:

Look at the picture of "en" in Fig. 18.6, page 666, and think about how all the intervening single bonds between the lone pairs on the outside N's contribute to considerable flexibility of the "en" molecule; accordingly, it can position the lone pairs in an octahedral arrangement around the central ion. See Fig. 18.13 for a related structure, involving a different central ion. Does that help?

Question:

Professor, I would like to know if you will be giving us a problem set that will be similar to the Final Exam like you have with the past two Midterms. Thanks

Answer:

Yes. I will try to have it ready by Wednesday, along with a short set of answers.

Question:

Professor, could you post the assignment for next week or at least the Chapters to read before Friday? I'd like to do them now so I won't be worrying about them during the weekend. Also, for the finals, do we have to memorize the different functional groups? Thanks.

Answer:

Sure. I'll be asking you to read pages 659-675 -- but don't worry there about reduction potentials or equilibrium constants, both of which will only be treated NEXT quarter. Read, also, pages 851-874, for "light", qualitative, appreciation. Then work problems 8, 16, 20, 22, 24, 52, 54, Chapter 18 AND problems 14, 16 (I made add a few more) Chapter 25

Question:

Why is the the triplet state lower in energy than the the singlet state?

Answer:

Because unpaired electrons are able to "stay farther away from eachother", hence experiencing less coulomb repulsion; but I don't know how to derive this on an elementary level.

Question:

hi professor Gelbart would u please post the extra problem on voh. i am not clear about what we are suppose to find, "K" or vibrational energy? thank u

Answer:

Sorry, I just got around to doing this late yesterday afternoon. We can talk about it in office hours, too.

Question:

Prof. Gelbart, I sent you an email on Monday about missed homeworks. Could you please please consider my request and reply my mail?

Answer:

Sorry, I just got to answering your email late yesterday afternoon.

Question:

Professor, I need my exam to be regaded, and I'm not sure when to give it to you. I'd like to explain my answer to you, and in office hours, there are a ton of people there. What can I do? Also, can you also still regrade the first midterm?

Answer:

I have asked that regrade requests be submitted to me with a written note; then we can talk about it AFTER I have had a chance to sit with it. Yes, it is definitely too late to consider a regrade of the first midterm. Yesterday, 1pm, was the deadline for the 2nd midterm, but if you can leave yours with me by this afternoon I will accept it.

Question:

When is Problem Set #8 due ?

Answer:

Friday, Dec. 3.

Question:

Will the answers to the second midterm be posted on the website?

Answer:

Yes, they should be there by Wednesday, Dec. 1.

Question:

On question #52 in chapter 16, we are given experimental facts about Be2's bond length and dissociation enthalpy. Then we are asked to compare the experimental bonding data on Be2 with those recorded for B2, C2, N2, and O2. The question is whether or not the prediction that stems from the simple theory seriously incorrect. How can we deduce this when all the facts that we are working with are experimental? When does the prediction that comes from the simple theory take a part in this problem? Even the ground state molecular configuration is given in table 16.2. Is there anything more to this problem than what meets the eye?

Answer:

It's true that you appear to be given all the answers in this Table. But the idea is to reconcile a simple MO prediction of NO bond for Be2, say, with the observed, experimental, bond energy of 9kJ/mole; and the increase, and then decrease, in bond energies as one goes through the given series. Want to talk about it at office hour?

Question:

Prof. Gelbart, I didn't read the syllabus properly and I've always thought that the homework problems aren't counted, but just for practice. I've not been consistent with it, and now that I found out homeworks are worth a whopping 20%, I'm wondering if you'd give me a chance to make up for the homeworks I missed. I realise it's my error for not reading the syllabus properly and not regularly attending discussion. I'd understand if you're not willing to give concessions. Thank you for your consideration though. Vilia

Answer:

Sorry, I cannot make an exception for you. But I would be glad to help you with learning the material if you come to office hours.

Question:

Professor Gelbert, I recently had an accident and am unable to use my writing hand. As a result, I am unable to take the second midterm, and need to be excused. I would like to know what would happen in the event of a missed midterm. Thank you for your time, Christopher Woon (section 1M)

Answer:

Sorry about your hand, and your not being able to take the exam today. We'll have to assign you a grade for it that is based on your performance on the corresonding part of the final. Good luck with your hand.

Question:

Prof. Gelbart- I have a question regarding the second question on problem set #7. At the review session, you discussed the IE(NA +) and the EA(F -) and how they fit into calculating the bond energy. Does this account for the fact that NaF isn't an ionic bond? If so, then in problem 2, wouldn't the bond energy be solely dependent on the coulombic attraction, since we were told to assume a 100% ionic bond?

Answer:

The 100% here refers to the fact that we assume that a full electronic charge (e) is transfered from one atom (Na) to the other (F). But this would imply too large a dipole moment -- remember our discussion in the first lectures on this stuff, on the 1st and 3rd of November?

Question:

How can we tell (without using experimental data) that in H2O, the oxygen is sp3 hybridized? Why couldn't it be sp hybridized? Is it because there would be better overlap, or because of the steric number??

Answer:

Because of the steric number.

Question:

I've been sick and did not go for lecture last friday. I'm wondering if you mentioned in lecture, how many questions there'll be on the midterm and roughly what areas the questions will cover. Thanks.

Answer:

Yes. There will be three questions, covering ionic bonds, and bonding in diatomics and triatomics (molecular orbital theory). Remember that the exam is in Moore 100, from 7 to 9pm tonight, Tuesday. Problem Set #7 will be handed in at lecture on Wednesday. Hope you're feeling better!

Question:

I have a question regarding problem number two on problem set #7. The book indicates that the dissociation energy of a bond is positive, yet you did an example during class one day and you said that the dissociation energy of a bond is negative. Also, I was wondering if the molecular orbitals on the NF2 filled differently than those on the O3 molecule due to the difference in electronegativity between sodium and fluorine.

Answer:

First: dissociation -- or bond -- energies are always indicated as POSITIVE quantities, just like ionization energies. Possible confusion arises because the actual energy of the electrons in the bond is NEGATIVE, compared to what it would be when the atoms are separated (just like the energy of a bound electron in an atom is negative compared to its energy when it is separated from the nucleus). Second, about NF2+. You are right to worry about the order of the orbital energies here, as compared with O3, but it's hard to talk about via email. Please bring up these questions at the review session this afternoon. Thanks

Question:

Professor, for problem number three on problem set 7, we need the 1s wave function for a hydrogen atom, but I have been unable to find this in the book. If you could tell me where it is or post it on the web page I would appreciate it.

Answer:

Table 15.2 (p. 554), which we have consulted many times, no?

Question:

In atoms such as oxygen, how can we tell what hybridization the molecule has when bonding? It seems to me that there could be sp, sp2, or sp3 hybridization.

Answer:

Yes. Which particular one of the hybridization schemes is operative, if any, depends on the situation. E.g., sp2 in O3 and sp3 in OH2.

Question:

When is Homework #7 due?

Answer:

Wednesday, at the beginning of lecture.

Question:

is the 2nd midterm gonna be cumulative or just from the stuff after the 1st midterm? also how come in ur example of the molecular orbitals of O3 there are (2p)2 orbitals and a (2sp)2 orbital instead of more pi orbitals? How does co2 have all it's remaining electrons in pi bonds but o3 doesn't

Answer:

Because CO2 is linear, it possible to form an "extra" set of pi orbitals (of the bonding, nonbonding, and antibonding type). Please bring up these questions in the review session, so that we can discuss them further.

Question:

Prof. Gelbart- I have two questions. 1) Is IE(F) equal to -EA(F minus)? 2) In the molecule KF, why is E(infinity) equal to IE(K)- EA(F)?

Answer:

EA(F)=IE(F minus). (There's no "-" sign in this relation, since EA is defined as the energy RELEASED upon adding an e, i.e., as the NEGATIVE of the energy CHANGE. Is that clear?) IE(K) - EA(F) is the work we have to do to transfer an electron from K to F when they are separated by an infinite distance. Catch me after lecture to talk more about it, or bring it up at the review session on Monday evening.

Question:

Prof.Gelbart i was trying to construc the correlation diagram of Oxygen3(-)but instead of getting twelve molecular orbitals, i got 13.The central atom O has a sp3 hybridization and has 19 valence electrons. But when i was trying to figure out the molecular orbitals i was unable to get 12 orbitals. if its not too much trouble can you please explain what i'm doing wrong. thank you for your time.

Answer:

It's hard for me to figure out what you have done wrong, without seeing your work and discussing it -- so why don't you try to catch me right after lecture on Monday, or bring up your questions at the review session Monday evening, or at my Tuesday 11am office hour. I will probably discuss O3- in lecture on Monday, and will argue that we still want sp2 hybridization for the central O.

Question:

Professor, what is the value for #L and #S for the electron affinity of Ne. I am confused as to where it goes after F, having 4 e's in #L and 5 e's in #S, when another electron is added. Thank You.

Answer:

In calculating the EA of Ne, we are basically calculating the IE of Ne with one e added -- do you understand why this is so? Hence we need Zeff for the outermost e in the negative ion of Ne. This e has 10 e's in lower-energy orbitals -- 2 in 1s, 2 in 2s and 6 in 2p -- so #L is 10, which is equal to Z, the bare charge on the nucleus of this ion. Hence, we predict an electron affinity of zero!

Question:

Are sigma bonds always localized and pi bonds always delocalized? If not how do you tell when they are?

Answer:

Pi bonds are not always delocalized. To figure out what is happening, one has to work carefully through the construction of molecular orbitals from atomic orbitals.

Question:

prof. gelbart... hwk set 6 is due friday? why is the key on voh already?

Answer:

Because we goofed! And then only the first pages of the solutions were posted, for sets #5 and #6! Sorry about that.

Question:

When bonding 2 atoms together, one with a 2sp2 hybridization, and the other with open 2py and 2pz orbitals, can the atoms have 2 sigma bonds together by putting one lobe of the 2py with a 2sp2 orbital and one lobe of the 2pz orbital with another 2sp2 orbital? If not, how would the bonds be configured?

Answer:

No. If the sp2 orbitals are formed from mixing 2s with 2px and 2py, then all the sp2 orbitals lie in the xy-plane -- the plane of the (bent, linear, triatomic) molecule. Accordingly, by symmetry, none of the sp2 orbitals from the central atom can overlap with a pz from an end atom. Does that make sense to you?

Question:

prof.gelbart, i was wondering if i can take my chem midterm a day before or a day after Nov 23rd...? Because i have 2 other midterms scheduled on the same day ....and i'm really confused about preparing for all 3 midterms on the same day... I will drop by during your office hours tomorrow morning, since i also have some questions on the chapter. Thank you, francisca

Answer:

Hi. I didn't answer earlier because you indicated you'd come by to my office hour this morning. About the midterm, I'm sorry but (even though -- or perhaps because -- many students have made requests similar to yours) I'm going to stick to my policy of no rescheduling or makeups of midterms. So please just do the best you can -- everybody is under similar pressure now. At least this time you will have all the time you need (i.e., up to two hours) to work the test.

Question:

Prof. Gelbart- In the molecule BF(3), as in problem 18a) on page 628, which orbital in each of the fluorine atoms overlaps with the hybrid orbitals in B. Is is it one of the 2p orbitals?

Answer:

Yes, because the p orbitals have better overlap along the bond direction (i.e., along the direction of the sp2 hybrids of the Boron).

Question:

Are you going to have a review for the midterm, if so where and when? Also the exam is on Tuesday at 7-9pm but what room? Thanks for your time.

Answer:

Yes, the review session will be Monday, from 5-6:30pm, in Moore 100. And the exam will be Tuesday, from 7-9pm, also in Moore 100.

Question:

Prof. Gelbart- On p.597, in Figure 16.16, the pi(nb) orbital has an energy between the pi* orbital and the pi orbital. Does this mean that electrons start filling into the pi(nb) orbital after the pi orbital is full, but before the pi* orbital?

Answer:

Yes, precisely. One seeks to organize the electrons into the lowest total-energy situation, and therefore fills up orbitals progressively, in order of increasing energy.

Question:

Hello, Professor Gelbart. I just wanted to know if you have allowed us to turn in this week's homework next week instead of this Friday. Thanks.

Answer:

As I will announce in class today, I think it's best to have you hand in Problem Set 6 on Friday, so that you can concentrate over the weekend and Monday and Tuesday on the special set of problems I asign for reviewing the past few weeks of material.

Question:

On page 63, the dissossiation energy is given by negative the coulomb energy of attraction minus the difference of the ionization energy and the electron affinity of the two ions. Conceptually, since the coulomb energy of attraction is negative, taking the negative of it will be positive, indicating that energy must be put into the molecule to break this attractive force. However, subtracting the difference of IE and EA (subtract the IE and adding the EA) indicates energy being given off in the ionization process and energy being needed for the electron affinity process. Doesn't this contradict the fact that energy is given off when an atom gains an electron and is needed when an atom looses one? Thank you for your time.

Answer:

I think you are being confused by the fact that the bond dissociation energy -- let's call it D here -- is defined as the NEGATIVE of the total energy released when the bond is broken. So: (1) the more negative the coulomb energy, the bigger D; the more positive the IE (corresponding to needing to supply more energy to create a postive ion), the SMALLER D will be; and the more positive the EA (the more energy released upon creating the negative ion) the BIGGER D will be. And the relation D = - (coulomb energy) - [IE - EA] is indeed consistent with all of this. Let's talk about it, if this is not clear.

Question:

Hello Professor Gelbert, Well, I just wanted to know if it was possible to take the 2nd midterm on Wednesday instead of Tuesday. I have to study for two other midterms (Math and Physics) that same day. I guess that's it. See you in lecture.

Answer:

Sorry, but I can't make any exceptions, because then I would have to make a LARGE number of exceptions....

Question:

why do the pi2p orbitals get filled before the sigma2p? also, why is the converse in true in diatomic oxygen and the elements at the end of the second period?

Answer:

Because the pi2p electrons are less "bothered" (electrostatically repelled) by the lower-lying 2s electrons. For high enough Z (i.e., for O, F and Ne), the 2s electrons are held closely enough to their nuclei so that they don't get in the way enough to disfavor the sigma2p any longer.

Question:

Is the pi*2p orbital lower in energy than the sigma*2p orbital?

Answer:

Yes, but only sometimes! E.g., for 2nd row elements with low enough atomic number. Do you understand why?

Question:

In the extra problem that you assigned, please explain what Rz (in particular z) reffers to inside the absolute value function in 1sB wave function.

Answer:

Rz (where z is the unit vector pointing along the positive z-axis) is the position of the right-hand nucleus! It follows that |r-Rz| (where r and z are both vector quantities -- but I can't show that with the limited word-processing ability of this email system) is the DISTANCE of the position r (vector) from the right-hand nucleus.

Question:

Concerning heteronuclear diatomic molecules, I noticed that on page 591 paragraph 2 that many things were taken into consideration when determining whether there will be a bonding, antibonding, or nonbonding orbital. "In the HF molecule, for example, the 1s orbital of the F atom is far too low in energy to mix with the hydrogen 1s orbital, and the same is true of the 2s F orbital." How are we to know this fact? Since all we have to work with is the correlation model and some general trends in ionization energies and electron affinities, how are we to know when the 1s orbital of one atom is sufficiently close to the 1s orbital of the second atom in order for a bond to occur? Another question I had was that in figure 16.9 (b), Nonbonding has occured because the phases cancel. When such a phenomenon occurs in the homonuclear diatomic molecules, an antibond is said to have occured and a node exists. Why the discrepency? Thank you for your time.

Answer:

You CAN'T know for sure when one atomic orbital is too low in energy to bond with its "corresponding" orbital in the other atom of a heteronuclear bond. But you can take some educated guesses and explore the consequences of each. Let's talk in person about this point -- it's an important one. As for your other question, I can't quite follow what you are asking, but I think I know what's confusing you -- again, talking in person would clear things up most efficiently.

Question:

On the extra problem, should we leave our answers in terms of a(0) (a-knot) and constant "C"? Or should the answer just be in terms of constant "C"? If that doesn't make sense, I'll ask you later. Thank you

Answer:

The answers can be in terms of the constant ("C"), but you can substitute distances in units of ao and end up with actual numbers for everything else (i.e., other than the multiplicative constant ("C")). Come by to my office hours, so that we can make certain you understand what I have in mind for this problem.

Question:

PRof. Gelbart....my name is danmy...i was wondering if i can schedule a one-on-one appointment? thank you danmy loi

Answer:

How about just after my office hour tomorrow, Thursday, i.e., at about noon? Otherwise, just after my office hour on Friday, i.e., at about 10am?

Question:

On the extra problem that you assigned I was wondering if r is considered negative when it is outside of the nuclei. It seems impossible to have have a negative distance, but if the distance is not negative then it is the same as if the electron was in between the nuclei, and this doesn't make sense either. Thanks for your help.

Answer:

We are always concerned about the DISTANCE between a given point and a particular nucleus, so -- in that sense -- r is always positive. For a point a distance "a" to the left of the left nucleus, for example, the relevant distances to the nuclei are "a" and "3a" (recall that the distance between the two nuclei is "2a"); for a point in between the two nuclei, the relevant distances are "a" and "a"! Does this make sense to you?

Question:

Question #12 deals with the compound nitrogen oxide (NO). When we make a correlation diagram for this molecule do we use the structure for Z<=7 or Z>=8? These two structures are located on page 588. Thank you for your time.

Answer:

There is no way, without recourse to experiment and/or more sophisticated theory, to know which correlation diagram pertains to the particular case of NO. So you have to consider each of the two possibilities, and make your predictions about bond order and magnetic properties for each!

Question:

Prof. Gelbart, in problem set #4, question 11 & 12 the book tells us to look up the Ionization energies and electron affinities for the elements. Will we be supplied the same information during the exam or will we have to be able to calculate these energies on our own?

Answer:

I will either give you the experimental values, OR ask you to estimate them from our simple Zeff model. Does that make sense to you?

Question:

Has our next midterm definitly been move from Nov. 24th to Nov. 23rd from 6:30 to 8:30PM. Thanks You,

Answer:

Yes!

Question:

Professor Gelbart, what is the chemical formula for a Perchlorate ion in problem 22 (d)?

Answer:

It is given as the last entry at the bottom of the list in Table 3.1, p. 62 of the text.

Question:

Prof. Gelbart, were you still going to post the answers to the even problems on VOH, as you said in class?

Answer:

Ooops, sorry, I forgot. We'll get them up as soon as possible now, but it won't be before Monday.

Question:

Hi, my name is Robert Ly and I would like to schedule a one-on-one appointment if that is possile. Thank you for your time.

Answer:

How about just before my 11am office hour tomorrow (Thursday), say at 10:45am?

Question:

My problem is that I wrote down the underlined questions off the board as 12, 20,22,24, and 42a+c. Whereas online it does not mention which ones are mandatory. I was wondering if we had to do just the ones listed above or all of them to hand in? Thank you.

Answer:

Just the underlined ones need to be handed in. Sorry about the confusion.

Question:

In the problems that you posted for this week (set #4), none of them are bold faced, or underlined. Does this mean we need to turn in all of them, or are there specific prolems you would like us to turn in?

Answer:

The voh office failed to include underlining of the following problems in assignment #4: 12, 20, 22, 24, and 42a&c. Sorry about that!

Question:

I was unable to make it to class today. What is the assignment for next week?

Answer:

Reading: pages 55-84 in Chapter 3. Problems: 2b&d, 12, 18, 20, 22, 24, 28, 38, 42a&c, and 44b&d, all at the end of Chapter 3.

Question:

Prof. Gelbart, how does your equation for Z(eff) account for the electron repulsion inside the same orbital (like 2px)? You said that the IE is proportional to Z(eff). In the book, O has a slightly lower IE than N because O has two electrons in the same 2p orbital. But in your equation, this trend is not evident.

Answer:

Good! You are right to observe that our simple model does not explain the slight decrease in IE from N to O, for the reason you suggest, i.e., #s -- "the number of other electrons in SAME-energy orbitals -- does not distinguish between single or double occupancy. But that's ok! Ours is a SIMPLE model, and it DOES correctly predict/explain the OVERALL increase in IE as one moves across a row. P.S. Be careful: IE is proportional to the SQUARE of Z(eff).

Question:

Professor Gelbart, I was wondering when the midterm grades would be available. Thanks

Answer:

Not until Monday, November 1. But the solutions will be posted by tomorrow, Friday, October 29.

Question:

Professor Gelbart, Could you please post your solutions to the problems that were on the midterm exam. Thank you.

Answer:

Yes, we will try to have them posted by tomorrow, Friday, October 29.

Question:

Prof. Gelbart, when we had to estimate the size of a hydrogen atom, did we have to find its volume or did we just have to find its diameter? I was unsure about the question and what you specifically wanted for the homework. thank you.

Answer:

EITHER volume OR diameter would be fine, since one tells us immediately the other, right?!

Question:

Professor Gelbart, I have been very sick this past week, and especially this weekend. I was unable to study at all for this midterm, or any of my other classes. I am still not completely well, and I read in the syllabus that if I have a valid reason for not talking the midterm, you would grade me according to how well I do on the final for that material that was covered on the midterm. I have a doctor's note for you to see, I can show it to you before class. I feel that I will not be able to perform to my potential if I take the test tomorrow. Please respond at your earliest convenience. Thank you for your consideration. Cordially, Kristen Coury 702846414

Answer:

Sorry I didn't get your message until this morning. I hope you are feeling better now. If NOT, then come by as soon as you can once you're well so that we can talk.

Question:

On the bottom of page 539, regarding ionization energy, when it reffers to the ground state, it says that n=1. We always talked about the ground state being denoted by n=0. Please clear up this matter. Thank you.

Answer:

No, we do NOT always talk about n=0 for the ground state. In fact, it is very important that n=1, not 0, for the ground state of the H-atom, and for the particle-in-a-box, etc. We might write a subscript "o" for ground-state properties, but n still equals 1 for the ground-state value of the quantum number.

Question:

is there anyway i can be excused for the midterm due to a cold and a painful broken arm? Please get back to me as soon as possible :jcspeedy88168@aol.com

Answer:

If you are too sick or uncomfortable to take the exam, then obviously you cannot take it. In a case like this I would assign you a grade for this midterm based on how you do on the second one, and on the final. I hope you are not getting behind in the work. And good luck with your arm!

Question:

hi, i have a question about degeneracy of the atomic orbitals. isnt it true that all the electrons with the same number of n are degenerate? so why do we say that for example, Es2

Answer:

It is ONLY in hydrogen-like atoms, i.e., ones in which only one electron is present, that we have degeneracy of all orbitals belonging to the same n value. As soon as two or more electrons are involved, shielding effects arise and the degeneracy is "broken", e.g., 2s becomes lower in energy than 2p because its electrons see a larger charge on the nucleus, etc.

Question:

Are you going to provide us with a page of equations on the mid term? I'm confussed on your comments during lecture regarding not having to memorize anything. Thank you.

Answer:

You will NOT have to have memorized anything. I'll explain more in lecture today (Monday).

Question:

Prof. Gelbart- I was wondering as to the format of the questions on the test. Are there going to be short answer questions on the themes in the text or just problems? Thank you.

Answer:

"Just" problems. I'll say a few words about the test in lecture today (Monday).

Question:

I am having trouble with problem #80 on problem set #3. I have gotten the radius of the sphere which I got from the equation for a sphere, and then I plugged the radius into the equation given squared, but I get a strange answer with a 10 to the -6 involved. What am I doing wrong?

Answer:

What's wrong with a probability that is of the order of 10 to the -6? Let's talk about it.

Question:

I was wondering where to begin on problem #5. Do we need to find the other products of the equation to balance it? Or is there another method to start this question?

Answer:

You don't need to know the other products, because NONE OF THEM CONTAIN CARBON. So start by assuming that all of the moles of carbon in CO2 must be accounted for by the two carbonates that react. Let x be the number of moles of one carbonate, and y of the other, and then set up two equations in these two unknowns....

Question:

I assume only chapter 15 is being covered in the midterm?

Answer:

Yes, but there will also be a stoichiometry problem, as I'll explain in lecture on Monday.

Question:

Hi, what I meant as cheat sheets are those little postcard-like little paper where we write down formulas, etc. but since you're letting us bring in textbooks, i guess cheat sheets are redundant. will the midterm be similar to the questions in the book? thanks.

Answer:

You won't need (or be allowed!) ANY notes or books at the exam. I'm confident you'll recognize the problems there to be similar to ones we have already worked and discussed, from homework and the extra problems from Friday.

Question:

you mentioned that we should be comforable with both notations we discussed for table 15.2 Can you clarify which numbers are assigned to the different "m" (quantum number which tells the orientation of l), say px is -1; py is 0; pz is 1, or px is 1; py is o, pz is -1, so on...

Answer:

Good question. It turns out that only m=0 can be assigned directly to one of the usual p-orbitals, i.e, pz. It's a little more complicated with m=1 and m=-1 -- they involve sums and differences of px and py. I don't expect you to be clear about this point; I don't consider it important for the course. But of course I would be glad to discuss it further with you if you are interested.

Question:

Never mind, Professor Gelbart. I understand now. Thanks though. Celine

Answer:

ok!

Question:

Professor Gelbart: On #1 of extra problems you assigned today, I was wondering what you meant by not using any of the numbers not already specified. Does that mean we can't use any constants or conversion factors(such as 1 rydberg = 2.18 * 10E-18 J? Can you clarify this for me? Thanks

Answer:

I meant that you don't need to start plugging in values of e and h, etc.; instead of calculating the n=5 energy "from scratch", you want to simply use the n-dependence of the general expression for the allowed energy. Let's talk about it!

Question:

Prof. Gelbart......i am having a hard time understanding the marterial in chapter 15...i was wondering if you had books that you would recommend on the topic that would make it easier to understand .

Answer:

Yes, you are welcome to borrow a book from me; just stop by my office after class, or during one of my office hours. Sorry I didn't respond sooner.

Question:

are we allowed CHEAT sheets for the midterms and exam?

Answer:

I'm not sure what you mean by a "cheat sheet". What I allow for exams is just the textbook, without any notes. Does that answer your question?

Question:

Hi Professor Gelbart, regarding the radial wave functions on page 554, table 15.2, what does the "exp" mean? I'm trying to do problem 44 but I can't solve for r because I don't know what to do about the exp part of the function.

Answer:

"exp(...)" is just the exponential function, i.e., e -- the base of the natural logarithms -- is raised to the power (...). Stop by if you are still not clear about this.

Question:

The Problem Set #3 webpage says that the third homework is due on Friday, October 25, 1999. October 25, 1999 is a Monday, not a Friday. When is the third homework assignment due?

Answer:

Sorry about the confusion. In any case, as I explained at the start of today's lecture, problem set #3, along with a few additional problems I will give you on Friday the 22nd and this coming Monday the 25th, will not be due until Wednesday, October 27th (i.e., it will be handed in along with the examination that day).

Question:

Our beloved Mr. Gelbart: We had difficulty solving problems 43, 44, and 80 of the latest problem set. Question: do we actually need to isolate r in 40? we came up with such a complex formula: log6-1 = -2/3 + logr. isolate r? i don't think so. i am kinda getting the others now... just this isolation of r that kept me awake until 1:00 am right now!!! Hope to see you in your office hours...

Answer:

I don't follow what you mean by "isolation of r". In problem 44, for example, you are asked to find the specific values of r for which the 3p and 3s radial functions vanish. By "isolating r" do you mean finding those values? Let's talk about it in office hours....

Question:

Could you please explain more clearly the section on the sizes and shapes of the orbitals? Thank you.

Answer:

Yes, this is a difficult section, and not one that can be easily explained in a few lines of text. So I am going to devote a good deal of time to these questions in my next lecture. Also in my office hours this week, so try to come.

Question:

Professor, there is a word "exp" in Table 15.2 in the test book, what does that mean?

Answer:

"exp" refers to the exponential function, e.g, exp(-s) means that e (the base of the natural logarithms) is raised to the power -s, and so on.

Question:

i'm having a lot of trouble understanding this whole concept of quantum mechanics and quantum theory because we barely convered it in high school chem. can you recommend any texts that can explain it simply yet adequately?... something along the lines of quantum theory for dummies or something? i think it would be very helpful... thank you

Answer:

Don't worry. I EXPECT EVERYONE to be having a lot of trouble understanding quantum mechanics because: it is new to you; and it is very difficult material. As I explained in lecture today, I will be elaborating on the material from Chapt. 15 throughout the rest of the course, and so you will have many opportunities to understand things better. If you want to look at another textbook, you are welcome to borrow one from me -- just stop by.

Question:

Where are the answered questions? They do not appear at the page linked by the VOH page for this class- instead of a cgi script like "unanswered questions," it just goes to a regular html page. Apparently, there is not yet a dynamic page for answered questions...

Answer:

Sorry; the answered questions didn't appear earlier because there WERE none -- I am only getting around today (10-15-99) to answering them....

Question:

About #26 from Problem set 2: Is potential energy at all involved in this problem, or is it just kinetic energy? In other words, is it necessary to calculate the PE of both n=5 and n=3, then subtract to find the change in PE? Or am I supposed to calculate the kinetic energy difference, assuming that the electrons are traveling at v=3E8 m/s?

Answer:

No, potential energy is NOT explicitly involved in this problem. Nor do you want/need to be thinking about ELECTRONS -- instead, it is LIGHT which is involved in the transitions from n=5. Stop by and we'll talk about it.

Question:

Professor Gelbart, I don't understand how to calculate the temperature required to ionize an atom. there was some discussion on it in section 15.2 but i think that that was referring to classical physics and that it has fundamental errors. can you please explain how to find the temp. and how the temp. can ionize an atom? thanks

Answer:

Since we haven't discussed kinetic theory (Chapter 4) yet, I won't hold you responsible for this problem (part (c) of problem 24 in Chapt. 15). I'll be glad to discuss it with you in an office hour, however, if you like.

Question:

On problem 15.24 you are asked to solve for the wavelength of light. I have values for all the variable except for the frequency of the electron (from the kenetic energy of the electron). How do you find this frequency?

Answer:

I don't seem to follow your question. Can you stop by and explain it to me? I'll be glad to help.

Question:

I was wondering if you were planning on posting some type of answer key for the supplementary problems assigned so that we can check our work.

Answer:

No, because I continue to feel that the very best way for you to learn is by talking directly with me or one of the teaching assistants. Sitting with solutions/answer keys, one can easily (but mistakenly) be persuaded that one understands the problem, instead of struggling further to discuss it.

Question:

Would you further explain Blackbody radiation because I did not understand the definition in the textbook

Answer:

As I have explained in lecture, I would like to DE-emphasize blackbody radiation, because it involves so very many topics that are beyond the scope of this course. I prefer to emphasize instead the photoelectric effect, the Franck-Hertz experiment, line spectra, etc., as examples of the need for developing a non-classical theory.