|MLA Citation:||Bloomfield, Louis A. "Question 618"|
How Everything Works 17 Jul 2018. 17 Jul 2018 <http://howeverythingworks.org/print1.php?QNum=618>.
The number of covalent bonds that form between two atoms depends on the number of electrons in those atoms. Hydrogen atoms have only one electron each and can form only one covalent bond. Oxygen atoms have two electrons each that they can share and form two covalent bonds. Nitrogen atoms have three electrons to share and form three covalent bonds. And carbon atoms have four electrons to share, so you might expect them to form four covalent bonds. But there's a hitch...
In the first covalent bond that forms between two atoms, the pair of electrons positions itself directly in between the atoms. This arrangement is most effective for lowering the energy of the system and binding the two atoms together. Chemists call this arrangement a "sigma bond." In the second covalent bond, the two electrons position themselves on both sides of the sigma bond. If you picture the atoms as two people facing one another and holding hands, the electrons are located along the arms of the two people. This arrangement is reasonably effective for lowering the energy of the system and is called a "pi bond." The third covalent bond is also a pi bond, but it forms 90° from the first pi bond, as though the two people are now touching their heads and their feet together along with their hands. With a sigma bond and the two pi bonds between the atoms, there is no room for additional electrons. The fourth covalent bond that two carbon atoms would like to form with one another simply can't form. While two carbon atoms will bind together with a triple bond, each atom will have one remaining electron that is still seeking a partner. The carbon dimer molecule is a highly reactive double radical that will bind to just about anything it encounters.