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It helps to take into account the 3D shape of the molecule rather than just the Lewis structure, as the Lewis structure may lead us to believe it is nonpolar when it is actually polar, and vice versa. Sometimes I will look at the symmetry of the forces acting on the molecule, because if it is symmetrical it is most likely nonpolar since the forces cancel out.
Yes. You should memorize which molecular shapes are symmetrical, and thus non-polar assuming that the dipoles cancel, and which one's are not. Symmetrically shaped molecules will tend to be nonpolar, asymmetrically shaped molecules will tend to be polar. Hope this helps!
The shape of the molecule and the difference in dipoles are also important to figure out whether a molecule is polar or non-polar. If the dipole forces cancel out then the molecule is non-polar.
I also like to see what the shape is in order to determine if it is polar or nonpolar. A lot of times, you can determine polarity by looking at if there are lone pairs or not. If there are lone pairs, it is usually polar.
Understanding the 3D model of the lewis structure is important too, as 2D images may be misleading. A common example is the linear molecular geometry from a trigonal bipyramidal electron geometry (AX2E3). While it may appear to be polar because of the asymmetrical lone pairs, it is still considered to be a polar molecule.
A good thing to do if you have time is to draw the lewis structure of the formula and check to see if the bonds are polar and nonpolar to see if it all cancels out. If the central atom however, it surrounded by atoms that are all the same in symmetrical positions then all the polarity should cancel out.
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