Wize University Chemistry Textbook > VSEPR Shapes and Polarity
Molecular Polarity
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Polar Covalent Bonds
Polar Covalent Bonds:
- Electrons are shared unequally between 2 different non-metals
- There is a difference in EN, 0.4 < ΔEN < 1.7
Since electrons are shared unequally in this bond, we say that there is a "dipole moment"
- The dipole moment is a vector with both magnitude and direction (more on this later!)
- Partial negative charge (δ-) is assigned to the atom with the higher EN
- Partial positive charge (δ+) is assigned to the atom with the lower EN
- The greater the difference in electron density (EN), the greater the dipole moment!
Wize Tip
You can think of the dipole moment as a tug of war. The dipole moment (arrow) points towards the winner that is able to pull electrons more towards itself (is more electronegative!)
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Molecular Polarity
How can we figure out if a molecule is polar or not?
Recall we talked about polar covalent bonds earlier, where electrons are shared unequally between 2 elements.
To figure out if a molecule is polar, we need to consider all of the bonds and use our knowledge of VSEPR.
Wize Concept
If there are no polar bonds in the molecule-> it is non-polar!
If they are polar bonds but they are symmetrical, their dipole moments will cancel each other out-> molecule is non-polar!
If there are polar bonds that are not symmetrical, the dipole moments don't cancel each other out and we are left with a net dipole moment-> molecule is polar!
Example: Polar Molecule
Example: Non-Polar Molecule
Example #1: AlCl3
Are there polar bonds? Yes!
Yes!
Is there an overall dipole moment? No!
No!
Therefore, AlCl3 is (polar/non-polar): non-polar
non-polar
Wize Tip
In order for a MOLECULE to be considered polar, then there must be a net dipole moment and the dipoles can't all cross out!
Example #2: Is H2O a polar molecule?
Are there polar bonds? yes
yes
Is there an overall dipole moment? yes
yes
Therefore, H2O is (polar/nonpolar): polar
polar
Example #3: Is NH3 polar or non-polar?
Are there polar bonds?
yes
Is there an overall dipole moment?
yes
Therefore, NH3 is (polar/nonpolar) : polar!
polar!
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Example: Net Dipole Moment
Determine whether the following molecules have a net dipole moment (polar molecules). For the ones that do, draw the direction of the net dipole moment.
a) XeF4
The molecular shape is square planar. The lone pairs on opposite sides so they cancel out.
The molecule does not have a net dipole moment and is therefore non-polar.
b) CCl4
The molecule does not have a net dipole moment and is therefore non-polar.
c) CHCl3
The molecule has a net dipole moment and is therefore polar
d) SCl2
The molecule has a net dipole moment (that points towards the 2 lone pairs of electrons) and is therefore polar.
Practice: Polar Molecules
Which of the following are polar? (Select all that apply) and indicate the direction of polarity.
Practice: Polar vs Non-Polar
Are the following compounds polar or non-polar?
PF5