What makes a Molecule Polar? Dipole?

In summary, polarity in molecules is based on the difference in electronegativity between atoms and the shape of the molecule. Symmetrical shapes, like tetrahedral or square planar, tend to be less polar while linear molecules can be polar depending on the distribution of electronegativity. Non-polar molecules have equal bond lengths and equal electronegativity between atoms.
  • #1
Whalstib
119
0
Hi,
1st year chem guy here...I'm missing this idea. Is it all based upon Electronegativity?

I can see why HBr or HCl and other 2 atom compounds would be polar. 2 atoms with different charges or even sizes creating an un-even pull.

I'm guessing H2O is polar as the 4 remaining electrons on the oxygen molecule create an imbalance.

I'm guessing CO2 is non polar because the O=C=O balances perfectly negating/balancing the charges...

But if you throw CCl4 at me I can find a definition that it is non-polar but couldn't show the math etc to explain it...

Our textbook explains it thusly:
"the greater the electronegativity difference, the more polar." And I understand the Pauling scale how differences determine either ionic or covalent

But HBr for instance has an electronegativity difference of 0.7 and is polar, yet CO2 difference is 0.5 and is non polar. That's pretty close. Wait a sec...is it because the CO2 molecule is balanced with one O on either side? Would CO be considered polar as it is just 2 atoms?

Could I assume ALL 2 atom compounds (ionic or molecular) are polar!?

Thanks,

Warren
 
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  • #2
What is the shape of the CCl4 molecule?
 
  • #3
Like you said, it's based on electronegativity, but shape plays a huge role in polarity. You were right, CO2 is non polar because of the symmetrical shape of the molecule. The dipoles cancel each other out. CO, on the other hand, would be polar because there isn't a second bond to cancel out the first. All bonds between atoms of different electronegativities are slightly polar. Non polar usually only means that the polarity is small enough to be ignored.

Here's the problem with your proposed rule. When you have a diatomic gas (O2, N2, etc.), the two atoms have the same electronegativity, so the elecrons are shared evenly, making it non polar. In this case, non polar really does mean absolutely non polar. Does this make sense?
 
  • #4
sjb-2812 said:
What is the shape of the CCl4 molecule?

Tetrahedal.

So is one shape more apt to be polar or enhance any electromagnetic differences?

Would one expect linear molecules to be polar? Except in cases of H2, O2, N2 etc...

Warren
 
  • #5
pzona said:
Like you said, it's based on electronegativity, but shape plays a huge role in polarity. You were right, CO2 is non polar because of the symmetrical shape of the molecule. The dipoles cancel each other out. CO, on the other hand, would be polar because there isn't a second bond to cancel out the first. All bonds between atoms of different electronegativities are slightly polar. Non polar usually only means that the polarity is small enough to be ignored.

Here's the problem with your proposed rule. When you have a diatomic gas (O2, N2, etc.), the two atoms have the same electronegativity, so the elecrons are shared evenly, making it non polar. In this case, non polar really does mean absolutely non polar. Does this make sense?

Thanks,

It makes sense...For some reason it's not explained so simply in our textbook. It's all mixed in with a discussion of electronegativity but the "rules" aren't really touched upon. I thought I had it before...

I'm working some excersizes now and if I find a weird one I will post.

Thanks,

Warren
 
Last edited:
  • #6
Whalstib said:
Tetrahedal.

So is one shape more apt to be polar or enhance any electromagnetic differences?

Would one expect linear molecules to be polar? Except in cases of H2, O2, N2 etc...

Warren


It's partly do with symmetry arguments, the more highly symmetrical a structure is, the less polar it is.

So tetrahedral structures, like carbon tet, or square planar species, like NiCl42- are less polar, than say, SF4
 
  • #7
Whalstib said:
Tetrahedal.

So is one shape more apt to be polar or enhance any electromagnetic differences?

Would one expect linear molecules to be polar? Except in cases of H2, O2, N2 etc...

Warren

Get a molecular model set and make a tetrahedral molecule (ex: CCl4). The bonds are all equally spaced and the 4 attached atoms all have the same electronegativity. Cl is more electronegative than C, but remember that they're all equally spaced which makes the molecular non-polar.
 
  • #8
Some molecules have polar bonds but that does not necessarily mean that the molecule is polar. The polarity of the molecule depends on the net pull of electrons (electronegativity) within the molecule. If the electronegativities of the peripheral bonds are equal and the bond lengths between the atoms are equal, the chances are it's non-polar. However, if there is an unequal distribution of electronegativity within the molecule the molecule will be polar. The more uneven the distribution, the more polar the molecule.
 

Related to What makes a Molecule Polar? Dipole?

1. What is a polar molecule?

A polar molecule is a molecule that has a partial positive charge on one end and a partial negative charge on the other end. This happens when the electrons in the molecule are distributed unevenly, creating a dipole moment.

2. How do you determine if a molecule is polar?

A molecule's polarity can be determined by looking at the electronegativity difference between the atoms in the molecule. If the difference is greater than 0.4, the molecule is considered polar. Additionally, the molecule's shape and symmetry can also affect its polarity.

3. What causes a molecule to be polar?

A molecule is polar when there is an uneven distribution of electrons due to the electronegativity difference between the atoms. This creates a dipole moment, with one end of the molecule being slightly positive and the other end being slightly negative.

4. Can a molecule be both polar and nonpolar?

No, a molecule cannot be both polar and nonpolar. A molecule is either polar or nonpolar based on its electronegativity difference and molecular geometry. However, some molecules may have both polar and nonpolar regions, making them overall polar.

5. How does molecular shape affect polarity?

Molecular shape can greatly affect a molecule's polarity. If a molecule has a symmetrical shape, the dipole moments of the individual bonds will cancel out, making the molecule nonpolar. However, if a molecule has an asymmetrical shape, the dipole moments will not cancel out, making the molecule polar.

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