[lillybeans]

Hey everyone,

So I am slightly confused now, because my teacher has taught me one method, and many internet sites (including Dept. of Chem at many universities) seem to agree with the second method. Please tell me which one is correct, thanks in advance

Method 1: My teachers'
- Find the type of bond that exists. If the molecule only has NONPOLAR bonds ( EN < 0.4), the molecule is NONPOLAR regardless of its geometry. According to this, O3, H2S are nonpolar, beecause EN difference is less than 0.4 between O=O (pure covalent) and H-S (nonpolar bond), even though both have bent shape.

Method 2: The rest of the internet
- If a molecule has a polar shape/geometry, then the molecule is polar regardless of having non polar bonds. According to this, O3, H2S are polar molecules because they have bent shape, even though their bonds are non polar.

So which one is the correct one? I checked, O3 and H2S do have a non-zero dipole moment, but does that make it polar? WHERE DO WE DRAW THE LINE between polar and non polar? Is there a definite boundary?

Help is much appreciated,

Lilly

[Schrödinger]

Isn't a polarity the same as having a non-zero dipole moment? Any molecule that is polar has an electric dipole moment

[lillybeans]

Isn't a polarity the same as having a non-zero dipole moment? Any molecule that is polar has an electric dipole moment

True, but consider the molecule BrCl. The EN difference is 0.20 (very tiny), which makes it a non polar molecule (EN < 0.40) HOWEVER, there is still an EN difference between Br and Cl, no matter how small it is, therefore the molecule still has a net dipole moment. But, does that make BrCl a polar molecule?

[nj_bartel]

By definition, yes.  There's nothing qualitatively different about a molecule with a .39 dipole moment and one with a .391 dipole moment.  The cutoffs for polar/nonpolar are just lines drawn in the sand for nomenclature purposes

[lillybeans]

By definition, yes.  There's nothing qualitatively different about a molecule with a .39 dipole moment and one with a .391 dipole moment.  The cutoffs for polar/nonpolar are just lines drawn in the sand for nomenclature purposes

Good point, so are O3 and H2S polar or non polar molecules? They both have a dipole moment that's some distance away 0... Theoretically they should be polar, but my teacher said nonpolar bonds = nonpolar molecule (and they both have non polar bonds EN < 0.4). I'm not too sure what to do at this point...

[Schrödinger]

Nonpolar bonds need not mean non-polar molecule, because in case the molecule is bent, we also need to consider the effect of the lone pair of electrons. I don't know about this 0.4 rule, but I like to consider anything > 0 as polar, by definition.

[argulor]

By definition, yes.  There's nothing qualitatively different about a molecule with a .39 dipole moment and one with a .391 dipole moment.  The cutoffs for polar/nonpolar are just lines drawn in the sand for nomenclature purposes

Good point, so are O3 and H2S polar or non polar molecules? They both have a dipole moment that's some distance away 0... Theoretically they should be polar, but my teacher said nonpolar bonds = nonpolar molecule (and they both have non polar bonds EN < 0.4). I'm not too sure what to do at this point...

They're symmetrical, so they're nonpolar.

[BluePill]

I think polarity is more defined by net dipole moment. H₂S is not polar because it closely resembles water. Water is polar due to its unsymmetrical electron distribution. The dipole moment is going to the oxygen. Ozone does not have a symmetrical electron distribution because the exterior oxygen are pulling the electrons, a net dipole moment results,

[umangarora]

H₂S is polar. See, S has 6 valence electrons, out of these, 2 are shared with 2 H atoms. So , 4 electrons are left untouched, i.e. they did not participate in Bonding, and formed lone pairs. thus, the molecule is not linear, it has a bent shape. and as a rule, only symmetrical molecules have zero dipole and are non polar. Also, if dipole is > 0, it is polar. else its not. as for molecules with 2 atoms, its only non polar when the electronegative difference is 0, i.e. only in atoms like H₂,O₂,Cl₂.

The molecule looks like H-S-H. EN of S is > EN of H. So, electron cloud is shifted more towards S from both sides, giving it a small negative charge at S and small positive charge at both H atoms. Thus making it a dipole.
Hope this helps.

[opti384]

So the main point will be the definition of polar molecules. I agree with Schrödinger that a polar molecule is a molecule that has a net dipole moment of > 0. However, as lillybeans said, bonds, not the polarity of molecules, may be considered polar/nonpolar according to EN.

[MrTeo]

Another example: take CO₂, C=O bonds are clearly polar but the simmetry of the molecule causes μ to be 0. Yet if you look at the structure of dry ice you'll see that there are attractive forces between oxygen and carbon atoms that keep it together, which could be locally classified as electrostatic interactions, just like those of polar molecules.