Chemistry / Unit 4: Chemical Bonding
4.4 Bond Polarity and Molecular Polarity
Notes
Electronegativity is an atom's pull on shared electrons. The difference between two atoms decides the bond type.
- Difference less than 0.4: nonpolar covalent (equal sharing).
- 0.4 to 1.7: polar covalent (unequal sharing). The more electronegative atom gets a partial negative charge ($\delta^-$).
- Greater than 1.7: mostly ionic.
- Example: H (2.20) and Cl (3.16): difference 0.96, polar covalent. Na (0.93) and Cl: difference 2.23, ionic.
- A molecule is polar if it has polar bonds that do not cancel. $\text{CO}_2$ is linear, so its bond dipoles cancel (nonpolar). $\text{H}_2\text{O}$ is bent, so it is polar.
Practice
Using electronegativities H = 2.20 and O = 3.44, the O–H bond is:
$3.44 - 2.20 = 1.24$, which is between 0.4 and 1.7.
Which molecule is nonpolar even though it has polar bonds?
$\text{CO}_2$ is linear, so its two C=O dipoles point opposite ways and cancel.
In the polar bond H–F, which atom has the partial negative charge?
Fluorine pulls the shared electrons closer, giving it $\delta^-$.
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