In this section, we will look at electronegativity and polar covalent molecules. It is electronegativity that determines whether a molecule is either nonpolar or polar. Such terms will also be described here.
What is electronegativity? It is a measure of an atom's attraction for electrons in the covalent bond. You would be right if you said the halogen elements (fluorine, chlorine, iodine, etc) have some of the highest electronegativities, since they want to get one more electron to achieve an octet state. Look at the following periodic table and notice how electronegativities vary in a consistent manner across each column and row.
When two different atoms are covalently bonded, the atom with the higher electronegativity will attract the shared electrons stronger than the other atom can. This bond is known as a polar bond. As the difference in electronegativities of the atoms in a bond increase, so does its polarity. The more polar a bond becomes, the more ionic it is likely to be. You know that NaCl, table salt, is ionic; notice the difference in electronegativities, which is about 2.3. The following figure illustrates how the covalent and polar character increase. Typically, bonds with differences in electronegativities of less than 1.7 are said to be of covalent character.
Lets look at a polar molecule for an example. A good example is HCl, hydrogen chloride gas. The difference in electronegativities is about 1.0, suggesting polar character. At any point in time, the chlorine atom will attract the shared pair more strongly than the hydrogen atom; at this point, a dipole-moment exists within the atom.
Now look at methane, which has four equal hydrogen bonds. Because the symmetrical distribution of the polar bonds in the molecule cancels out the effects of the bond polarity, methane remains neutral. If we were to replace one hydrogen with a chlorine atom (forming chloromethane), the chlorine atom would attract the shared pair of electrons more strongly than the hydrogen opposite to it. Thus, the molecule becomes slightly polar.
Have you ever noticed that oil does not dissolve in water? A few things to remember are that most but not all organic compounds (compounds containing carbon and hydrogen) are nonpolar. Also, if we look at a water molecule, we can see that there is a lack of symmetrical electron distribution around the molecule. Because of this, water has been shown to be polar. We all know that salt dissolves in water. Why? Because like dissolves like, thus water, being polar, dissolves polar substances. Now we know why cooking oil simply floats on water and does not dissolve.
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