Difference between bond polarity and molecular polarity?

[UrbanXrisis]

what is the difference between bond polarity and molecular polarity?

 

[t!m]

Take for example BF_3.

Its structure is a central Boron atom single bonded to 3 Fluorine atoms. The molecule, as a whole, is not polar, because it is completely symmetrical, and thus one could not point to a more negative side. Each of those single bonds, on the other hand, is polar, because Fluorine is more electronegative than Boron.

 

[UrbanXrisis]

so does HCl have a polar or nonpolar "molecular bond"? I wrote down polar but I think it has to be nonpolar acording to what you said

 

[apchemstudent]

so does HCl have a polar or nonpolar "molecular bond"? I wrote down polar but I think it has to be nonpolar acording to what you said

Since HCl molecule is defined as a molecule that consist of a hydrogen atom bonded to a chlorine atom, and the bond is polar, this also implies that the molecule is polar.

 

[UrbanXrisis]

how is the molecule polar if the structure is symmetrical?

does this mean every polar bond must have be a polar molecule?

 

[apchemstudent]

how is the molecule polar if the structure is symmetrical?

does this mean every polar bond must have be a polar molecule?

For instance, for a bond to be polar, there must be an unequal distribution of electrons amongst the bonded pair, due to the unequal electronegativity. Usually the less electronegative element is in the center of the molecule. However, if the molecule is symmetrical, by vector-adding, the attraction of the electrons is "cancelled" out since a substance on the opposite end provides an equal attractive force , due to the symmetry, upon the electrons.

We can use CO2 as an example

O=C=O is a symmetrical molecule, but it consists of polar C=O bonds. However, due to the symmetry, the O on each end cancels out the unequal attractive forces on the electrons, and thus the molecule is nonpolar.

 

[UrbanXrisis]

all I know is that symmetrical molecules are nonpolar molecules. That means linear and tetrahedral bonds are considered nonpolar molecules. However, if the EN is between .5-1.7 then it has a polar BOND. So, back to HCl

H-Cl has a linear bond, that means it is a nonpolar molecule.
However, HCl is a EN difference over .5 which means it is a polar bond.

Why is HCl a polar molecule as you said?

 

[apchemstudent]

all I know is that symmetrical molecules are nonpolar molecules. That means linear and tetrahedral bonds are considered nonpolar molecules. However, if the EN is between .5-1.7 then it has a polar BOND. So, back to HCl

H-Cl has a linear bond, that means it is a nonpolar molecule.
However, HCl is a EN difference over .5 which means it is a polar bond.

Why is HCl a polar molecule as you said?

because that's all the molecule is made of, just the linear polar bond of hydrogen and chlorine. There's nothing there to counter-act the electronegativity difference, unlike in a symmetrical molecule, and thus the molecule is polar. And by the way, being a linear molecule does not mean it will always be symmetrical, for instance HCl molecule. In general, if the opposite sides do not resemble each other, then the molecule should not be symmetrical (this is going on a limb here, but it works in most cases)...

 

[UrbanXrisis]

what about H2CO? it's a polar molecule and also has a polar bond right?

 

[apchemstudent]

what about H2CO? it's a polar molecule and also has a polar bond right?

You are correct.

 

[UrbanXrisis]

would this molecular structure be called bent or pyramidal? or is it called something else?

 

[apchemstudent]

would this molecular structure be called bent or pyramidal? or is it called something else?

Did your teacher explain to you about the different molecular geometries? I'll help you out. Well there's 0 nonbonding pairs and 3 bonding pairs... There's your hint...

 

[t!m]

Oops, sorry if I spoiled that one. Answer in white.

The structure of H_2CO would be trigonal planar, with 120 degree bond angles.

And in all fairness, apchemstudent, there are 0 nonbonding pairs and 4 bonding pairs, as the carbon double bonds to the oxygen. More accurately, the defining characteristic of the molecule is that there are 3 regions of electron density.

 

[apchemstudent]

The structure of H_2CO would be trigonal planar, with 120 degree bond angles.

great job... way to help someone learn...

 

[UrbanXrisis]

HCN has a linear bond, so that means that it is a nonpolar molecule. However, how did I know if the bond is polar or not? since H-C is polar and C=N is nonpolar, what would be the bond polarity?

 

[t!m]

Linear bonds are not always nonpolar. I think you're overgeneralizing the concept of symmetry. HCN is actually a polar molecule. Also, both bonds are polar as well, which is actually the reason for the molecule's polarity. Try http://onsager.bd.psu/~jircitano/polar.html site.

 

[UrbanXrisis]

why is C=N a polar bond? It has an EN of .4, doesn't that make it nonpolar?

 

[t!m]

Ack, this sort of thing really depends on teacher. No bond is truly nonpolar except for bonds between identical atoms, i.e. C-C bonds. If you're following the rules for END, then yes, I suppose, C=N is a nonpolar bond.

 

[Joel]

I can not give you the kind of in-depth answer many here can... But try this:

In high school I learned there are three kinds of bounds: covalent bounds, polar covalent bounds and ionic bounds. Between some electronegativity (EN) values the bound is called covalent, in others, polar covalent or ionic. This classification is good to know because it roughly predicts solubility, reactivity and polarity - ionic bounds are polar and covalent bounds tend not to be polar.

However, according the all mighty quantum mechanical model the orbitals (where the electrons reside) are in fact "electron probability densities" - at a higher density an electron is more likely to be found. Hence, according to this model, no electron swapping really takes place in an ionic bound, the density of electrons just becomes higher close to one of the bounding atoms. The electron density is in fact slightly higher close to the N atom than it is close to the C atom, because of the small, yet existing, difference in EN. That is probably why t!m said it was polar. (But isn't the CN bound actually a triple bound, in HCN? I think the lewis structure would suggest that...)

When thinking of the molecular polarity of HCN, in addition to the bound polarities and molecular structures, there is one more nifty (but not necessary, i presume) thing to keep in mind when determining the magnitude of molecular polarity. Since the the EN values increase from H to C to N, the CN bounds should be slightly more polar in the HCN molecule than by it self. (And therefore the HC bound should be slightly less polar?) That is because the electron density of the HC bound is so close to the C atom that it will repel the electron density in the CN bound further towards the N atom.

Now I hope I have not made a complete ass of myself with my first post and confused you, urbanxrisis even more. So, please ask what you've missed or correct whatever I've missed!

 

[r3dxP]

Ack, this sort of thing really depends on teacher. No bond is truly nonpolar except for bonds between identical atoms, i.e. C-C bonds. If you're following the rules for END, then yes, I suppose, C=N is a nonpolar bond.

Yes. C=N bond is nonpolar, but once you bond it with hydrogen ion, H+ and CN- forms together and becomes polar. Due to the positive and negative attraction in between these two ions, Hydrogen ion(the positive end of the polar vector) reaches, or becomes pulled towards the CN- ion(also known as the negative end of the polar vector which attracts the positive end).
Also, this can be seen when you draw this molecule using vector analysis.

-> = single bond(attracted to the right), ---> = triple bond(attracted to the right)

H -> C --->N

since hydrogen is attracted to carbon, we have a bond polarity going to the right. If this molecule was to be a nonpolar molecule, we need another vector line going inward towards the carbon from the same direction with the same length(length is determined by the attractiveness + bond type between the atoms). In our case, we have another vector line going to the right. Thus, does not cancel out the dipole moment, and making it have a permenant dipole moment. Hope this helps anybody. Correct me if I am wrong anybody~ thanks.

 

[r3dxP]

Linear bonds are not always nonpolar. I think you're overgeneralizing the concept of symmetry. HCN is actually a polar molecule. Also, both bonds are polar as well, which is actually the reason for the molecule's polarity. Try http://onsager.bd.psu/~jircitano/polar.html site.

To be specific of what T!M is implying, linear bonds with the same bond type/strength would be always nonpolar. The example given by T!M, HCN, is a polar molecule indeed because there is a single bond on the left, and a triple bond on the right. Thus, cannot balance these two different bond types. On top of that, according to the vector analysis, both of the vector lines are facing right, and cannot cancel each other out, making the HCN polar. O=C=O would be nonpolar(linear) because of the equalivalent attrativeness/polarity between the O=C and C=O.