Some molecules are polar when the atoms are bonded together. This is due to a difference in the electronegativity levels. An EN difference from about 0.5 to 1.7 will make a molecule polar covalently bonded.
Polar bonds happen when there is an EN difference between 0.5 to 1.7. One atom pulls electrons closer to it than the other atom, which makes them orbit around that atom more often. This creates two poles, one slightly positive and one slightly negative. In many cases, the existence of these poles (dipoles) makes the molecule a polar (dipole) molecule. In the example of water below, the oxygen atom (red) is the negative pole; the hydrogen atoms (white) are centers of positive charge. Because a oxygen to hydrogen bond creates a polar bond, the oxygen attracts electrons to it more and creates a center of negative charge.
In some other cases, two polar bonds can cancel each other. Take the example of carbon dioxide, shown below in the right.
Because both C-O bonds are polar, they create centers of positive and negative charge. However, since the molecule's structure is linear, the bonds are at direct opposites of each other. The attractive forces of each in either direction cancel each other's out. The molecule, therefore, has no center of charge, and is not a polar molecule. To the right, CH2O has a polar bond. Since it is not canceled out by an opposite force, it is a polar molecule overall.
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