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A covalent bond represents a shared electron pair between nuclei. The
Stability of covalent
bonds is due to the build-up of electron density between the
nuclei. Using Coulomb's
law (discussed in Ionic Bonding),
you should note that
it is more stable for electrons to be shared between nuclei than to be near
only one nucleus. Also, by
sharing electron pairs nuclei can achieve octets of electrons in their
valence shells, which leads
to greater stability.
To keep track of the number and location of valence electrons in an atom or
molecule, G. N. Lewis
developed Lewis structures. A Lewis structure only counts valence
electrons because
these are the only ones involved in bonding. To calculate the number of
valence electrons, write
out the electron configuration of the atom and count up the number of
electrons in the highest
principle quantum number. The number of valence electrons for neutral
atoms equals the group
number from the periodic table. Each valence electron is represented by a
dot next to the symbol for
the atom. Because atoms strive to achieve a full octet of electrons, we
place two electrons on
each of the four sides of the atomic symbol. Some examples of Lewis
structures for atoms are
shown in .
Figure %: Lewis structures for atoms
We can create bonds by having two atoms come together to share an electron
pair. A bonding pair of
electrons is distinguished from a non-bonding pair by using a line between
the two atoms to represent a bond, as in the figure below. A lone pair is
what we call two
non-bonding electrons
localized on a particular atom.
Figure %: Lewis structure of HBr
You should note that each atom in the H-Br molecule has a full valence
shell. Both the
hydrogen and the bromine can count the two electrons in the bond as its
own because the electrons are shared
between both atoms. Hydrogen needs only two electrons to fill its valence,
which it gets through the
covalent bond. The bromine has an octet because it has two electrons from
the H-Br bond and six
more electrons, two in each lone pair on Br.
The deadly gas carbon monoxide, CO, provides an interesting example of how
to draw Lewis
structures. Carbon has four electrons and oxygen has six. If only one
bond were to be formed
between C and O, carbon would have five electrons and oxygen 7.
Figure %: Lewis Structure of CO
A single bond here does not lead to an octet on either atom. Therefore, we
propose that more
than one bond can be formed between carbon and oxygen so that we can give
each atom an octet of
electrons. To complete the carbon and oxygen octets in CO, we must employ
a triple bond, denoted
by three lines joining the C and O atoms as shown in . A
triple bond means
that there are six electrons shared between carbon and oxygen. Such
multiple bonds must be
employed to explain the bonding in many molecules. However, only single,
double, and triple
bonds are commonly encountered.
