(Polar molecules, Non-polar molecules, etc.)
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Some times you can determine if a molecule is polar or not by its Lewis Structure; however the VSEPR model is the most accurate to help determine the polarity of a molecule. By looking at the shape and determining the arrangement of the atoms in the molecule, you can then tell if it is polar or not. Generally, asymmetric molecules, molecules with dipoles that do not cancel, and molecules with odd numbers of different atoms are polar. Symmetric molecules tend to not be polar.
In order to determine if a molecule is polar or non-polar, you can draw out the lewis structure and identify the dipole moments of the given molecule. If the dipole moments cancel, the molecule is non-polar. If the dipole moments do not cancel, the molecule is polar.
A way that you determine whether or not a molecule is polar from looking at the Lewis structure is determining the dipole moments (if there are any), and if it does have dipole moments that do not cancel out then the molecule is polar.
The way that I mentally do this is in my head add up all the valence electrons, then divide by 8. If the total number of v-electrons is not a factor of 8, there must be Lone pairs. When ever you have 1 pair of lone pairs, the molecule will be polar. Be careful because some lone pairs can surround molecules and not change its shape. Ex: if there are 3 pairs of lone pairs on a linear molecule then it will still be linear, it may also still be polar (it depends).
The way I look at it is to envision the shape and look at its symmetry. Usually non-symmetric shapes with lone pairs are polar, and symmetric shapes without lone pairs or have lone pairs that cancel each other out (as in the example of a square planar molecule) are non-polar.
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