3.14: Covalent Bonds

Another way that an atom can fill its outer electron shell is to share electrons with other atoms. Sharing of electrons between neighboring atoms creates a very strong and stable bond called a covalent bond (covalent in Latin means strong together).* A water molecule (H₂O) has two hydrogen atoms that each form a covalent bond with oxygen. By sharing electrons, the two hydrogen atoms and the oxygen atom fill their outer electron shells (Figure 3.1).

When 2 atoms share 1 pair of electrons (1 electron from each atom), it’s called a single bond. When 2 atoms share 2 pairs of electrons, it’s called a double bond. Scientists draw a single line between the symbols for atoms when the 2 atoms are linked by a single bond; a double line means a double bond. In Figure 3.1, for example, a single bond between the 2 hydrogen (H) atoms in hydrogen gas is shown as H-H, and the double bonds are shown as O=C=O in carbon dioxide.

Structures of carbohydrates, fats, proteins, vitamins, and even genes are based on arrays of carbon atoms linked together by covalent bonds. It’s no coincidence that carbon atoms make up the basic structure of the complex molecules that are unique to life. Carbon plays this central role because it has 4 electrons in its outer electron shell (Figure 3.1). Since this shell needs 8 electrons to be complete, carbon forms 4 covalent bonds with other atoms (4 pairs of electrons = 8).

This ability to form 4 stable bonds with a wide variety of atoms gives carbon the versatility to form an immense variety of structures, from simple molecules like carbon dioxide (CO2) to complex molecules like cholesterol (C₂₇H₄₆O). Carbon atoms combine to form the basic structure of carbohydrates, proteins, fat, and even vitamins.

Carbon’s versatility can be demonstrated with Tinkertoys, using rods as bonds and hubs as atoms—you can make a larger variety of structures with 4 rods per hub (e.g., carbon) than with only 1 (e.g., hydrogen), 2 (e.g., oxygen), or 3 (e.g., nitrogen). It’s no wonder that life on earth is carbon-based.

^{*Covalent bonds are formed when atoms share electrons to become more stable, the electrons orbit the protons in both atoms. To simplify, this isn’t shown for the covalent bonds in Figure 3.1, e.g., hydrogen gas should show the 2 electrons in a single orbit around both protons.}