polarizability and molecular mass

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I've read previous threads but still confused.

One of the explanation to the questions I did says London Dispersion forces depend on polarizability but it also says it should increase with molecular mass. Doesn't polarizability increase from R to L but molecular mass increases from L to R. So which is it? I'm talking about AAMC 4 Question 7. Thanks you

Originally Posted by yellowjellybean

I've read previous threads but still confused.

One of the explanation to the questions I did says London Dispersion forces depend on polarizability but it also says it should increase with molecular mass. Doesn't polarizability increase from R to L but molecular mass increases from L to R. So which is it? I'm talking about AAMC 4 Question 7. Thanks you

There is no trend for molecular mass in the periodic table. The periodic table covers atoms, not molecules.

As far as dispersion forces are concerned, polarizability and molecular mass both matter, but these effects are largely independent of each other.

The periodic trends are right to left(edit:this is not correct, I am just bolding this to point out my incorrectness) and down a column increase polarizability of atoms. This is due to decreasing effective nuclear charge as you follow those paths. This has nothing to do with mass since adding a neutron would not significantly affect the effective nuclear charge. Polarizability of the electrons in atoms is what allows a nonpolar molecule to have a temporary dipole in the first place.

Increasing mass of a molecule often means more atoms which means a greater surface area to the molecule. This allows for more/stronger interactions between molecules.

Between N2 and O2, N2 has the lower BP. Nitrogen is more polarizable but O2 has a higher molecular mass. Is it always the molecular mass that determines the stronger London Dispersion forces?

Originally Posted by yellowjellybean

Between N2 and O2, N2 has the lower BP. Nitrogen is more polarizable but O2 has a higher molecular mass. Is it always the molecular mass that determines the stronger London Dispersion forces?

Boiling points are not solely due to London Dispersion forces and London Dispersion forces are not solely due to a single aspect of a molecule.

Trends like knowing that larger molecules usually have higher boiling points and greater dispersion forces are useful, but there are no absolutes.

If O2's specific BP was due solely to dispersion forces and dispersion forces were solely due to mass, then F2 should have a higher BP than O2, but it doesn't.

N2 and O2 are both nonpolar molecules so that is why I am asking about London Dispersion forces.

I will try to phrase what I'm asking a little better. In reference to AAMC 4 #7, Why would O2 be more polarizable than N2 if N atoms are more polarizable than O atoms?

Thanks

Based on what the explanation the test gave, it looks like you depend on molecular weight to give you the highest bp if you're dealing with similar chemical species. If you think about N's ability to H-bond like O, and form many similar parallel compounds that O does, N is similar enough to use this rule. The same would apply for elements in the same group. When talking about elements in the same group, generally higher molecular mass increases polarizability due to increased number of electrons, and therefore would have a higher bp. This is why polarizability increases as you go down a column (AW increases). And N and O are similar enough that it too follows the molecular weight standard. It's probably why if you were to compare 2 alkanes, molecular weight would again play the most important role since we are comparing two similar species.

I see, thank you!

Edit: Doesn't it say the heavier species is more polarizable?

Originally Posted by yellowjellybean

I see, thank you!

Edit: Doesn't it say the heavier species is more polarizable?

Yep, and O2 is heavier than N2, hence is more polarizable since they behave similarly.

The explanation you cited does not fit with actual trends in mass versus boiling points for F2, O2, and N2.

In terms of mass, N2<O2<F2
In terms of boiling point, N2<F2<O2

Obviously there is more going on than just a mass difference.

I looked up the problem, and of the answers offered, the one they selected is probably the most significant factor, but their explanation is not a very good one given the real data.

Originally Posted by V5RED

The explanation you cited does not fit with actual trends in mass versus boiling points for F2, O2, and N2.

In terms of mass, N2<O2<F2
In terms of boiling point, N2<F2<O2

Obviously there is more going on than just a mass difference.

I looked up the problem, and of the answers offered, the one they selected is probably the most significant factor, but their explanation is not a very good one given the real data.

Eh...if your'e talking about my explanation, albeit it was rather a simple one, I don't think it's purely dependent on mass difference for the obvious example you provided. You can use mass to approximate bp's of differing compounds if these compounds in question behave similarly to one another. And O behaves quite differently to F, so mass cannot be used to determine relative bps of each.

If they behave differently, then more information is needed because polarizability doesn't have a definite trend on the periodic table from what I've seen so far.

Originally Posted by Jengreef

Eh...if your'e talking about my explanation, albeit it was rather a simple one, I don't think it's purely dependent on mass difference for the obvious example you provided. You can use mass to approximate bp's of differing compounds if these compounds in question behave similarly to one another. And O behaves quite differently to F, so mass cannot be used to determine relative bps of each.

If they behave differently, then more information is needed because polarizability doesn't have a definite trend on the periodic table from what I've seen so far.

I wasn't talking about your explanation, but I don't think it is very good either.

Fluorine can form H-bonds just like nitrogen and oxygen. Fluorine can be acidic or basic just like nitrogen and oxygen. Also, generally you talk about elements in a group(column) as having similar chemical properties, not ones in separate groups.

I think this link has some decent explanations.
http://www.newton.dep.anl.gov/askasc.../chem00935.htm

edit: I will say that I think that this question is a very poor question, but at least you can probably eliminate the "incorrect" answers and get the question "right"

Ah, fair enough. I did look at that page, and formed my response based on the info there and the explanation given in the solutions, but you do have a point about the correlations between F and O. But since the answer stated that N and O were similar chemical species, and since this idea of polarizability being most predictable among such species was brought up again in that second post of your link, I drew the conclusions that I did. Now, I don't know how reliable the explanations given for amcaas exams are or if they're necessarily all accurately explained, but I have assume they are unless someone here provides a better explanation for it.

But I do think your points are all valid, and that this was indeed a poor question.

Originally Posted by Jengreef

Ah, fair enough. I did look at that page, and formed my response based on the info there and the explanation given in the solutions, but you do have a point about the correlations between F and O. But since the answer stated that N and O were similar chemical species, and since this idea of polarizability being most predictable among such species was brought up again in that second post of your link, I drew the conclusions that I did. Now, I don't know how reliable the explanations given for amcaas exams are or if they're necessarily all accurately explained, but I have assume they are unless someone here provides a better explanation for it.

But I do think your points are all valid, and that this was indeed a poor question.

I think you misinterpreted the link.

They refer to using trends for atoms in the same group, not atoms that sometimes act in similar ways. Nitrogen and oxygen are in different groups, so they do not apply to the explanation of similar chemical species.

I was also wrong to say that polarizability increases from right to left. This is not necessarily the case as evidenced by the explanation at the link.

They then say that when comparing nitrogen, oxygen, and fluorine there are at least two factors in play to determine the polarizability and thus dispersion force. These factors are effective nuclear charge and number of electrons. Less effective nuclear charge means it is easier to push the electrons around so the atom is more polarizable. More electrons to push around also increases the polarizability. Nitrogen has a weaker effective nuclear charge, so its electrons are easier to push around, but there are less electrons to push around. Fluorine has a stronger effective nuclear charge, so its electrons are harder to push around, but there are more electrons to push around. Oxygen is in between these, and by chance this has imparted it with the greatest polarizability of the three.

This is not due to mass. This is due to charges and number of electrons. Mass also plays a role in boiling point, so it is useful for the question, but it is not a factor in polarizability. There is a trend of increasing mass correlating with increased polarizability in a group and increasing molecular mass increasing polarizability, but those are correlations, not cause and effect relationships.

Ah...good stuff.

Then between the two factors mentioned, would there be one that impacts polarizability more heavily? Or would you say that each holds equal impact?

If each holds equal impact, then this is a very troubling question since there is really no way to discern which element is truly more polarizable (had this not been a comparison of O F and N and instead had been of 3 other elements from different groups). Would you say at that point the only way to answer the question is to eliminate the wrong choices? Or is there another way to determine polarizability and dispersion force strength?

Come to think of it, even eliminating wrong choices in this particular question would've been unfruitful since, if molecular weight is truly an irrelevant factor to boiling point for such a scenario, then you'd have to cross out all four choices and you're nowhere further along than you were before.

Maybe I can say which factor matters more after I take physical chemistry lol. I am definitely not qualified to say which holds more weight or if there is even a trend.

I would say this question comes down to eliminating bad choices. The choices were reactivity, electronegativity, mass, and bonding(nitrogen having triple bonds and ocxygen double).

There is no reaction, so reactivity is out. Electronegativity refers to unequal sharing of electrons in a bond, and in all the choices there is perfectly equal sharing so that is out. Mass might work especially when you consider that at a given kinetic energy(ie temperature) a heavier particle will move more slowly. I don't know what effect bonding has on polarizability if any. It would come to a toss up between mass and bonding and my gut would lean to mass.

Overall, I think the question is awful and the explanation is just as bad. It is not unfair per se as you can pretty well narrow it down to two choices and give a slight edge to one, but otherwise it is not good.

think about polarizability as being a "sea of electrons" that shifts when subject to other electrostatic forces. this shift causes a small "dipole" so to say, which in turn can polarize other molecules in the proximity. this is basically what london dispersion forces are. the higher molecular weight means that there are more electrons, and if you have more charge in general the polarization would be larger. (the bigger the charge the bigger the force)

boiling point of a substance depends on the intermolecular interactions. the stronger the interactions are, the stronger the boiling point will be, which is why substances that hydrogen bond usually have much higher bps than other similar compounds.

Originally Posted by shaboobly

think about polarizability as being a "sea of electrons" that shifts when subject to other electrostatic forces. this shift causes a small "dipole" so to say, which in turn can polarize other molecules in the proximity. this is basically what london dispersion forces are. the higher molecular weight means that there are more electrons, and if you have more charge in general the polarization would be larger. (the bigger the charge the bigger the force)

True, but this reasoning breaks down when comparing O2 to F2. F2 has more electrons, but is less polarizable, and therefore has a smaller bp than O2.

Originally Posted by V5RED

I don't know what effect bonding has on polarizability if any

But wouldn't the number of bonds affect polarizability? If we're defining polarizability as the ease in which electrons are moving around, the increased number of bonds would allow more "channels" so to speak through which electrons from one atom can interact with another. This would increase the fluidity of the electrons and thus increase polarizability and bp. I might be misunderstanding polarizabilty here, but it was the reason I eliminated bonds as a possible choice because increased polarizability would be a reason for N2 to have a higher, not lower bp than O2.

Originally Posted by Jengreef

But wouldn't the number of bonds affect polarizability? If we're defining polarizability as the ease in which electrons are moving around, the increased number of bonds would allow more "channels" so to speak through which electrons from one atom can interact with another. This would increase the fluidity of the electrons and thus increase polarizability and bp. I might be misunderstanding polarizabilty here, but it was the reason I eliminated bonds as a possible choice because increased polarizability would be a reason for N2 to have a higher, not lower bp than O2.

You may be right. I just don't know. I am taking physical chemistry in the fall. Maybe I will know after that.

Originally Posted by Jengreef

True, but this reasoning breaks down when comparing O2 to F2. F2 has more electrons, but is less polarizable, and therefore has a smaller bp than O2.



But wouldn't the number of bonds affect polarizability? If we're defining polarizability as the ease in which electrons are moving around, the increased number of bonds would allow more "channels" so to speak through which electrons from one atom can interact with another. This would increase the fluidity of the electrons and thus increase polarizability and bp. I might be misunderstanding polarizabilty here, but it was the reason I eliminated bonds as a possible choice because increased polarizability would be a reason for N2 to have a higher, not lower bp than O2.

there are exceptions to almost every rule or pattern. take for instance how F is more electronegative than Cl, but HCl is by far a much stronger acid than HF, in part because F is small and the effective nuclear charge felt by the H is greater than the nuclear charge felt by H on the Cl. HF is indeed a weak acid..

Originally Posted by shaboobly

there are exceptions to almost every rule or pattern. take for instance how F is more electronegative than Cl, but HCl is by far a much stronger acid than HF, in part because F is small and the effective nuclear charge felt by the H is greater than the nuclear charge felt by H on the Cl. HF is indeed a weak acid..

That is a non answer. There needs to be a reason why their boiling points vary. The explanation of mass does not fit. The explanation of effective nuclear charge versus increasing number of electrons does.

On the topic of haloacids and electronegativity, the pattern for acidity of haloacids is HF<HCl<HBr<HI This trend is due to the size of the halogen, not its electronegativity. A larger halogen anion can better stabilize a negative charge than a smaller halogen anion. This is not an example of a trend with an exception.
http://chemwiki.ucdavis.edu/Organic_...y_and_basicity

Originally Posted by V5RED

That is a non answer. There needs to be a reason why their boiling points vary. The explanation of mass does not fit. The explanation of effective nuclear charge versus increasing number of electrons does.

On the topic of haloacids and electronegativity, the pattern for acidity of haloacids is HF<HCl<HBr<HI This trend is due to the size of the halogen, not its electronegativity. A larger halogen anion can better stabilize a negative charge than a smaller halogen anion. This is not an example of a trend with an exception.
http://chemwiki.ucdavis.edu/Organic_...y_and_basicity

you are missing the fact of the actual bond length and how electronegativity plays a part in that. since F is very electronegative the bond it makes with H is relatively weak and H should easily dissaociate, however F and H are both relatively small atoms which makes the bond length very small which is a reason why it is a significantly less acidic acid as compared to HCl.

HCl is far more acidic because of the fact that Cl, like F, is also pretty electronegative which makes for a weak bond. that fact also remains that Cl is SIGNIFICANTLY larger than F, making the bond more likely to disassociate. so it turns out that electronegativity does, in fact, relate to acidity because of the electronegative atoms ability to stabilize the negative charge, in the sense that they pull the electrons away (accepting negative charge).

So this is relevant to the original topic...how? If you want to discuss acidity of F compared to Cl, you should do so in a different topic and limit discussions to clarification of the original question here.

Originally Posted by shaboobly

you are missing the fact of the actual bond length and how electronegativity plays a part in that. since F is very electronegative the bond it makes with H is relatively weak and H should easily dissaociate, however F and H are both relatively small atoms which makes the bond length very small which is a reason why it is a significantly less acidic acid as compared to HCl.

HCl is far more acidic because of the fact that Cl, like F, is also pretty electronegative which makes for a weak bond. that fact also remains that Cl is SIGNIFICANTLY larger than F, making the bond more likely to disassociate. so it turns out that electronegativity does, in fact, relate to acidity because of the electronegative atoms ability to stabilize the negative charge, in the sense that they pull the electrons away (accepting negative charge).

The only effect of those that you listed that is of any significance with regard to the haloacids is the same one I listed, size of the halogen. The reason is because of the higher stability associated with spreading the negative charge through a larger anion as opposed to a smaller anion. More stable base=stronger acid.

Electronegativity is obviously not a significant factor since the expected trend based on electronegativity is the opposite of the actual trend. It matters with some acids like the oxoacids, but with haloacids it is irrelevant. The reason it matters with oxoacids is because the attached oxygens withdraw electron density from the protic element which leads to a more stable base.

The bond length of the haloacids has almost nothing to do with electronegativity. The bond length trends is because of the size differences which is again the main factor in haloacid strength.

Originally Posted by Jengreef

So this is relevant to the original topic...how? If you want to discuss acidity of F compared to Cl, you should do so in a different topic and limit discussions to clarification of the original question here.

You are right. I guess I am done with this topic, but it definitely was interesting to look into the polarizability trends and various factors involved with the boiling points.

Originally Posted by V5RED

The only effect of those that you listed that is of any significance with regard to the haloacids is the same one I listed, size of the halogen. The reason is because of the higher stability associated with spreading the negative charge through a larger anion as opposed to a smaller anion. More stable base=stronger acid.

Electronegativity is obviously not a significant factor since the expected trend based on electronegativity is the opposite of the actual trend. It matters with some acids like the oxoacids, but with haloacids it is irrelevant. The reason it matters with oxoacids is because the attached oxygens withdraw electron density from the protic element which leads to a more stable base.

The bond length of the haloacids has almost nothing to do with electronegativity. The bond length trends is because of the size differences which is again the main factor in haloacid strength.

the halogens were an example, not the main topic. electronegativity does have an effect in acidity because if the bonds are weak, then it would be more likely to disassociate. bond length also does have something to do. if the bond is longer, it means there is less energy involved in the disassociation making the substance more likely to release H+ into solution, which is the definition of acidity.

Originally Posted by Jengreef

So this is relevant to the original topic...how? If you want to discuss acidity of F compared to Cl, you should do so in a different topic and limit discussions to clarification of the original question here.

the discussion of polarizibility -> discussion of charges -> example of acidity -> more specific the halogens. it is all related.