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I'm still confused how to tell if a molecule is polar or nonpolar/ how to know which atom has the delta negative or delta positive charge. Is there a way you can tell which atom has the positive or negative charge by looking at the periodic table or something?
You can tell which atom in the molecule has the delta negative or delta positive charge by determining which atom is more electronegative. Based on the periodic trends we learned for electronegativity, you can determine which molecules would attract a bonding pair of electrons more. The more electronegative atom attracts the electrons more, giving it the delta negative charge.
Determining whether a molecule is polar or nonpolar requires a combination of knowledge of an element's electronegativity (which can be seen through trends on the periodic table) and an understanding of the shape of the molecule. For a molecule to be polar, is must have dipole moments-- at which atoms have differing electronegativities-- that do not cancel. For a molecule to be nonpolar, there must be zero dipole moments (i.e., the atoms have the same electronegativity such as an H2 molecule), or if there are dipole moments, they must cancel. They cancel when they are arranged symmetrically around the central atom, as in a CH4 molecule.
If dipole moments, caused by differences in electronegativity, do not cancel, then the molecule will be polar. The atom with the greater electronegativity will get the delta negative and the atom with the lower electronegativity will have the delta positive. Electronegativity follows the general trend on the periodic table of increasing from left to right and bottom to top.
It is based upon electronegativity, which is highest on the upper right of the periodic table. Non-polar molecules typically have a difference of 1.5 or less between the two atom's electronegativity values. Polar molecules typically have a 2.0 or greater difference. Symmetry and shape also play a part in determining polarity. If the molecule is symmetrical shape-wise with the same atoms on both sides, it is usually non-polar.
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