3
The second major type of atomic bonding occurs when atoms share electrons. As
opposed to ionic bonding in which a complete transfer of electrons occurs, covalent
bonding occurs when two (or more) elements share electrons. Covalent bonding occurs
because the atoms in the compound have a similar tendency for electrons (generally to
gain electrons). This most commonly occurs when two nonmetals bond together.
Because both of the nonmetals will want to gain electrons, the elements involved will
share electrons in an effort to fill their valence shells. A good example of a covalent bond
is that which occurs between two hydrogen atoms. Atoms of hydrogen (H) have one
valence electron in their first electron shell. Since the capacity of this shell is two
electrons, each hydrogen atom will "want" to pick up a second electron. In an effort to
pick up a second electron, hydrogen atoms will react with nearby hydrogen (H) atoms to
form the compound H
2
. Because the hydrogen compound is a combination of equally
matched atoms, the atoms will share each other's single electron, forming one covalent
bond. In this way, both atoms share the stability of a full valence shell.
There are, in fact, two subtypes of covalent bonds. The H
2
molecule is a good example
of the first type of covalent bond, the nonpolar covalent bond. Because both atoms
in the H
2
molecule have an equal attraction (or affinity) for electrons, the bonding
electrons are equally shared by the two atoms, and a nonpolar covalent bond is formed.
Whenever two atoms of the same element bond together, a nonpolar bond is formed.
A polar covalent bond is formed when electrons are unequally shared between two
atoms. Polar covalent bonding occurs because one atom has a stronger affinity for
electrons than the other (yet not enough to pull the electrons away completely and
form an ion). In a polar covalent bond, the bonding electrons will spend a greater
amount of time around the atom that has the stronger affinity for electrons. A good
example of a polar covalent bond is the hydrogen-oxygen bond in the water molecule.
Water molecules contain two hydrogen atoms Oxygen, with six valence electrons,
needs two additional electrons to complete its valence shell. Each hydrogen contains
one electron. Thus oxygen shares the electrons from two hydrogen atoms to complete
its own valence shell, and in return shares two of its own electrons with each hydrogen,
completing the H valence shells.
The primary difference between the H-O bond in water and the H-H bond is the degree
of electron sharing. The large oxygen atom has a stronger affinity for electrons than the
small hydrogen atoms. Because oxygen has a stronger pull on the bonding electrons, it
preoccupies their time, and this leads to unequal sharing and the formation of a polar
covalent bond.