[poobear]

Hi.

I am reading everywhere that CO2 is not considered polar. The same goes for e.g. CCl4 and BF3. The reason is that the dipoles cancel each other out. So if we take CO2. I understand that it doesn't have a dipole moment due to its linear structure. But it should still be polar? I.e. more negative in its ends than in its middle. I see a large distinction between polar and dipolar but I don't know if its correct or not.

[opti384]

No. CO2 is not a polar molecule. One thing I really think chemistry books should elaborate on more is that the dipole moment is a vector quantity. Force and velocity are examples of vector quantity.

[Honclbrif]

Like opti said: "dipole moment is a vector quantity". This means that you add the dipoles up, and the overall dipole is their sum. For CO₂:

<---+--->
O==C==O

You are right that there is more electron density at the ends than the middle, but think of the two vectors as equaling +n and -n. Though they may be large, they add to zero and perfectly cancel each other out.

[tamim83]

CO₂ is quadrupolar since it has positive charge in the middle and negative charges on the ends. 

[poobear]

opti384+Honclbrif: As I said, I understand that CO2 is not a dipole. But why is it not polar? The definition of polar, at least for me, is covalent bonds where the value in Linus Paulings electronegativity scale is more than ~0,4. And that is the case for CO2. So the ends of CO2 have a higher density of electron that the middle of CO2. Is it not this simple?

tamim83: At least someone agrees with me But shouldn't it be called tripolar and not quadrupolar? Since it has two negative regions and one positive region.

[opti384]

The definition for polarity is, from Wikipedia, "polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment."

For further information: http://en.wikipedia.org/wiki/Chemical_polarity

[poobear]

But why is it not a multipole? Because the electron distribution IS uneven in CO2.

http://en.wikipedia.org/wiki/Chemical_polarity
"In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment."
But then the article seems poorly written, e.g. "Example 4. The boron trifluoride molecule (BF3) has a trigonal planar arrangement of three polar bonds at 120o. This results in no overall dipole in the molecule." It doesn't say anything about multipolarity. I don't understand why so many people seem to use dipole interchangable with polarity.

[opti384]

Well it clearly says that the definition of polarity matters with the dipole moment which is a vector quantity.

Also, for BF3 it clearly states that "This results in no overall dipole in the molecule" because the dipole moment vector sum will be 0 for BF3.

Indeed since oxygen has a greater electronegativity than carbon there could be instantaneous dipole and induced dipole. However, when we talk about whether CO2 is a polar molecule or not it will be a nonpolar molecule because the definition of polarity considers the dipole moment.

[tamim83]

At least someone agrees with me  But shouldn't it be called tripolar and not quadrupolar? Since it has two negative regions and one positive region.

Well, I wasn't exactly agreeing with you, and I am sorry if I caused a lot of confusion.  Everything that the other posters told you is true, a polar molecule has a non-zero dipole moment vector sum.  Since CO₂ does not fall into this category, its not a polar molecule.  The fact that it is "quadrupolar" doesn't change this. 

Also, it is a quadrupole because the carbon atom would have a partial positive  charge that is twice the magnitude of the partial negative charge on each oxygen. Quadrupole and other "multipoles" are way above the level that you need to worry about. 

[poobear]

Thanks for all the explanation. I get the definition now.

A polar molecule is NOT what it sounds like, just being a molecule with polar bonds that give rise to polar regions in the molecule, i.e. regions with higher and lower electron density.

A polar molecule is a molecule with a permanent dipole moment. So regardless if CO2 have polar bonds that give rise to regions with higher and lower electron density within the molecule, it does not have a dipole moment, as the dipole moment is a vector quantity and the vectors from the two polar bonds cancel each other out.

[opti384]

Perfect.