Covalent Bonds

When two or more atoms with similar electronegativities interact, they achieve a noble gas electron
configuration by sharing electrons in what is known as a covalent bond. The binding force between
the two atoms results from the attraction that each electron of the shared pair has for the two
positive nuclei. This sharing of electrons is best envisioned by using dots to represent valence
electrons as follows:

Each fluorine atom has seven valence electrons; they are both one short of a stable octet. Unlike the
case of ionic bonding, however, there are no willing “electron donors” with low electronegativity
around from which they can grab an electron. What they need to do is to each share one electron
with its partner: The first structure drawn on the right-hand side of the arrow shows how each F
atom now has eight valence electrons; the pair in the middle is shared by both. This pair of electrons
is known as a bonding pair of electrons, as opposed to the unshared lone pairs, and is what
constitutes the covalent bond between the two F atoms in the F 2 molecule. The bonding nature of
these atoms is better indicated by the line between the atoms shown in the second structure.

Sometimes forming an octet requires sharing more than one electron from each atom. The oxygen
molecule, O 2 , and carbon monoxide, CO, for example, involve two and three pairs of bonding
electrons, respectively:

When two pairs of electrons are shared, the bond is known as a double bond. When three pairs of
electrons are shared, the bond is known as a triple bond. The number of shared electron pairs
between two atoms is called the bond order; hence a single bond (as in F 2 ) has a bond order of one,
a double bond has a bond order of two, and a triple bond has a bond order of three.