(Polar molecules, Non-polar molecules, etc.)
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What does it mean to have a dipole moment and how does this determine if the molecule is polar or nonpolar? Plus I dont understand when people ask if there is symmetry. Are we looking for the shape or if things cancel out?
If the dipole moments cancel out, then most of the time that means that there is symmetry. Dipole moments occur when atoms do not share their electrons equally, causing the electrons to be pulled one way. If dipole moments cancel out, then the molecule is nonpolar. If they do not, then the molecule is polar. You are more looking at whether dipole moments cancel out than the shape of the molecule, but you can use both to determine polarity.
So the shape of an atom will help you determine whether the dipole moments cancel out, and therefore whether the molecule is polar or nonpolar. You need to know the 3D geometry because there are some molecules that can be drawn in a Lewis structure to look "symetrical" but are actually not (such as CH2Cl2 which is tetrahedral). If there are dipole moments of the same value (same atoms) in opposite directions they will cancel out, otherwise there will be polarity in the atom.
When people talk about symmetry, it just means that if you were to fold the molecule evenly, it would be able to be equal on all sides, like O2 or CH4. In this case, molecules like these would be nonpolar because if they are symmetrical, they have equal pulls/charge distributions. On the other hand, polar molecules would be those that are not symmetrical, meaning their pulls are not equal. Dipole moments occur when within a bond, one atom is more electronegative than another and therefore gets a slight negative charge on it while the other gets a slight positive charge. The larger the electronegativity difference between the two atoms, the larger the dipole moment. Hope this helps!