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Covalent Bonds
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.
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