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How can molecules with polar bonds be nonpolar?
A molecule can have polar bonds, but it will be nonpolar if the bond dipoles cancel.
A bond dipole occurs when there is a separation of positive and negative charges in a bond.
Together, the positive "pole" and the negative "pole" make a dipole.
Chemists usually draw a bond dipole as a vector pointing from plus to minus.
To decide if a bond is polar, you need to draw the Lewis structure of the molecule.
Let’s use ##"CO"_2## as an example.
If a molecule has two polar bonds pointing in opposite directions, is the molecule polar or nonpolar? Check it out:
In this case, we can see that the C=O bonds are both polar, with oxygen pulling electrons away from carbon. But they are pulling equally in opposite directions.
The two bond dipoles cancel. As a result, the molecule is nonpolar.
Now let's look at the Lewis structure of ##"PH"_3##:
The big guideline to use to figure if a molecule is polar or not is this: Does the central atom of the molecule have more than one type of thing on it?
That is, does the central atom contain only one type of atom, or are there other atoms or lone pairs of electrons present.
In this case, the phosphorus atom contains both hydrogen atoms and a lone pair.
Since hydrogen atoms and lone pairs are clearly different things, the molecule is polar.