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 .
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.
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.
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.