(Polar molecules, Non-polar molecules, etc.)
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Dipole moments cancel each other out when they are equal in charge and opposite in direction. This can happen when a central molecule is surrounded by multiple identical atoms (such as in linear, trigonal planar, tetrahedral, or octahedral molecules like CH4 and SF6). In these structures, each surrounding atom exerts an equal pull on the central molecule's electrons (since they are identical and have the same electronegativity), and spread out to have angles as far apart as possible from each other. In this way, each atom's dipole cancels out another, creating a net dipole of zero on the molecule (thus making it nonpolar).
It depends on the shape of the molecule at hand. If the molecule is perfectly symmetrical, then forces of equal strengths will cancel out. If the molecule isn’t perfectly symmetrical then strengths will migrate to one direction. Examples of symmetrical shapes are tetrahedral and linear while examples of asymmetrical shapes are see-saw and trigonal pyramidal.
Another hint in figuring out whether a molecule is going to be polar is its geometry. If the same atom is spaced around the central atom such as CH4 then the electronegativities around the central atom will be equal in magnitude and symmetrical and thus be nonpolar; for shapes with multiple atoms, there will generally be at least a slight dipole because the different atoms will have different electronegativities.
For an example in which dipoles don't cancel out, look at CH3Cl. C is the central atom with 3 H's and 1 Cl around it. Since Cl is way more electronegative than H, the dipole moment (or partial charge) on Cl is greater than the partial charges on any of the H's. Since the electrons are unequally attracted to Cl, the dipole moments aren't equal, which means they don't cancel.
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