Polar Covalent Bonds - In some molecules, atoms share electrons unequally, developing areas that have slightly positive and negative charges, which form a polar covalent bond. When atoms form covalent bonds, they often share the electrons equally, so that the charges are equally distributed. However, in some special cases, the shared electrons are drawn to one section of the molecule because the electrostatic pull of that section is stronger. In these cases, the bond is called a polar covalent bond.
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| Polar Covalent Bonds |
For instance, think about the structure of a water molecule. Most of us already know that water is H2O two hydrogen atoms and one oxygen molecule. And we know that hydrogen has an atomic number of 1. Oxygen has an atomic number of 8, which means that it has 2 electrons in its first energy level and 6 electrons in its second energy level. In a water molecule, oxygen and two hydrogen atoms form covalent bonds in order to fill oxygen's second energy level and hydrogen's first energy level.
However, even though the three atoms share electrons in order to fill their energy levels, the oxygen nucleus, with its eight positively charged protons, has a stronger attraction for electrons than the hydrogen, with only one proton. As a result, the majority of the electrons are near the oxygen portion of the molecule, which gives that region a slight negative charge. (Remember, electrons have a negative charge.) The two hydrogens, on the other hand, have fewer electrons near them, so they have a slight positive charge. As you can see in Figure 2.11, the water molecule assumes a shape having four corners, with two positively charged regions and two negatively charged regions. In other words, there is an unequal distribution of charges in the molecule.
Because given portions of the molecule have differing electrostatic charges, the molecule is said to be polar, and the bonds are said to be polar covalent bonds. (The use of the word polar here is meant in the same sense as the term poles when you are dealing with magnets.) The polarity of the water molecule leads us to the discussion of the next type of bond that is important in biological systems, the hydrogen bond. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 45)
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