The Polar Covalent Bond: Examples And PicturesThe polar covalent bond, called a polar bond for short, is a variation on the standard covalent bond. It is defined by a difference in electronegativity values of 0.4 or greater, the meaning of which shall be made clear below. All covalent bonds are polar to some extent unless the bond is between two atoms of the same element. It is best to start with a review of the standard covalent bond. The Standard Covalent Bond This is the sharing of electrons between two elements in order to have 8 electrons in the outer shell. The only exception to this is Hydrogen, which is stable with 2 electrons in its outer shell. The structure of each element gives it a different electronegativity value. This value is effectively the strength of the pull of that atom's nucleus on the electrons around it. The higher the value the greater the pull. A covalent bond is electrons moving around two atoms; they are being shared. It is the difference between the electronegativity values that determines which atom gets the larger share of the electron's time. If the electrons spend more of their time around one atom out of the pair then that region will have more negative charge than the other atom. Examples Of Covalent Bonds Carbon to Carbon Bond The first example is the standard Carbon to Carbon bond such as occurs in the alkane molecules. We are just considering the bond that these two atoms share without regard for any other bonds that this pair of atoms may be involved in. First we can draw the two atoms as shown below. The pair of electrons that form the bond are drawn between them. The values written below the atoms are from the electronegativities table. The difference is calculated which in this case is zero.
This means that neither atom claims more share of the pair of electrons, so they spend equal time around each. If we draw the electrons as clouds of probable locations as theorized by Schrodinger, then the Carbon to Carbon bond would look like this:
Carbon to Hydrogen Bond If we set this up the same way as before we get this:
The difference of 0.4 in electronegativity values is small but will have some effect on the distribution of electrons. This bond is classed as polar, and if we draw this bond in cloud form it would look like this:
Oxygen to Hydrogen Bond This pair of atoms has a large difference in electronegativities, as can be seen in the following diagram:
This is a huge difference in electronegativity values. This will mean that the electrons will spend almost all their time around the Oxygen atom and virtually no time around the Hydrogen atom. In cloud form, the bond would look like this:
This unfair distribution of electrons has a very special effect. Since the Oxygen has the electrons around it for most of the time, it has a slight negative charge. It effectively has one more electron than its neutral state. The Hydrogen on the other hand loses out; it's nucleus is frequently exposed meaning that on the whole the Hydrogen carries a slight positive charge. It can be shown like this, where the squiggly s, called sigma, means "slight".
A polar covalent bond occurs every time Hydrogen bonds with Nitrogen, Oxygen or Fluorine as these are the three elements with the highest electronegativity values. They all have a difference of 0.9 or greater with Hydrogen. These bonds are called polar because of the different charges. These act like magnets and so polar molecules are pulled toward each other, with opposite charges attracting. The polar covalent bond is commonplace. Water is a liquid at room temperature because of these bonds. Ammonia (NH3) dissolves readily in water because of these bonds. This model even explains why water expands as it freezes. A polar covalent bond involving Hydrogen with any of the three most electronegative elements of Nitrogen, Oxygen and Fluorine is especially strong and is called a Hydrogen bond.
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