Covalent bonds form when two atoms share electrons. Hydrogen is a simple example of an atom that usually shares electrons with other atoms. As you can see in the figure below, a hydrogen atom has a single electron and an unfilled outer electron shell. A filled outer shell at this energy level requires only two electrons. When hydrogen atoms are close enough to one another, they form pairs, with each of their single electrons moving around the two nuclei. These paired atoms of hydrogen are called diatomic molecules (die-uh-TOM-ick) and are represented by the molecular formula H2. The figure below left depicts this sharing as two types of shell models and a structural formula (middle).
As a result of sharing electrons, the diatomic hydrogen gas molecule is electrically balanced because it now contains two protons and two electrons. In addition, each hydrogen atom has two electrons in its outer shell, completing this shell. This relationship also results in the pairing of two free electrons. Thus, by sharing their electrons, the two hydrogen atoms form a stable molecule.
Covalent bonds can be very strong, that is, difficult to break. Double bonds, those bonds in which two pairs of electrons are shared, are stronger than single bonds in which only one pair of electrons is shared. As you might expect, triple bonds, those bonds in which three pairs of electrons are shared, are the strongest of these three types of covalent bonds. In chemical formulas that show the structure of covalently bonded molecules, single bonds are represented by a single line between two bonded atoms, double bonds by two lines, and triple bonds by three lines. For example, the structural formula of hydrogen gas is H--H, oxygen gas is O == O, and nitrogen gas is N == N.
An atom can also form covalent bonds with more than one other atom. Carbon (C), for example, contains 6 electrons: 2 in the inner shell and 4 in the outer shell. To satisfy the octet rule, it must gain 4 additional electrons by sharing its 4 outer-shell electrons with another atom or atoms, forming 4 covalent bonds. Because there are many ways that 4 covalent bonds may form, carbon atoms are able to participate in many different kinds of molecules in living systems, such as proteins and carbohydrates.
The strength of a covalent bond refers to the amount of energy needed to make or break that bond. The energy that goes into making the bond is held within the bond and is released when the bond is broken. Therefore, covalent bonds are actually a storage place for energy as well as a type of chemical "glue" that holds molecules together. Living things store and use energy by means of making and breaking covalent bonds, thereby using molecules as a type of energy currency.