Covalent Bonding and the Octet Rule

Covalent Bonding and the Octet Rule

Hydrogen, with just one electron in its 1s orbital, can achieve a noble gas configuration (that of helium) by obtaining a share of one electron from another atom. The Lewis structure of H₂ implies that both atoms have access to both electrons in the bond.

Since hydrogen obtains a stable valence shell configuration when it shares just one pair of electrons with another atom, hydrogen atoms only form one covalent bond. (This should be easily confirmed from the organic unit as well).

The Octet Rule

The valence shell of the noble gases other than helium all contain eight electrons, and the tendency of many atoms to acquire such an outer shell electron configuration forms the basis of the octet rule.

Octet Rule: When atoms react, they tend to achieve an outer shell having eight electrons.

Many of the representative elements (sodium and chlorine for instance) follow this rule when they form ions. In the ionic case they achieve the noble gas shell configuration by either a gain or loss of electrons. When atoms other than hydrogen form covalent bonds, an octet is accomplished by sharing. The octet rule can be used to explain the number of covalent bonds an atom forms. This number normally equals the number of electrons that atom needs to have a total of eight electrons (an octet) in its outer shell. For example, the halogens (Group VIIA), all have seven valence electrons.

The Lewis symbol for a typical member of this group, chlorine, is
           ..
         :Cl:
           .
 

There is only one electron needed to complete an octet. Of course, chlorine can actually gain this electron when it becomes and ion. When chlorine combines with any other nonmetal, the transfer of electrons is not energetically favourable. Therefore, in forming such compounds as HCl or Cl₂, chlorine gets the one electron it needs by forming a covalent bond.

               ..                   ..              ..
     H� + :Cl: ------> H:Cl: --> H-Cl:
               .                    ..             ..
 

Multiple Bonds

The bond produced by the sharing of one pair of electrons between two atoms is called a single bond. This first bond is designated a sigma bond. There are however many molecules in which more than one pair of electrons are shared between two atoms. For example, we that nitrogen, is diatomic, N₂. Each N atom has the Lewis symbol of

       .
     .N:
       .
 

and each nitrogen needs three electrons to complete its octet. When the N₂ molecule is formed, each nitrogen atom shares three electrons with the other.

        �-------�
      :N�-----�N:
        �-------�

The result is called a triple bond. Notice that in the Lewis formula for the molecule, the three shared pairs of electrons are placed between the two atoms. We count all of these electrons as though they belong to both of the atoms. Each nitrogen therefore has an octet

           :N:::N: OR :N=N:

In the triple bond above the middle bond is a sigma type bond and the other two are Pi bonds. The Pi bonds are slightly different because they have to be bent out of shape in order to connect properly. But more on this later.

Double bonds are also possible. eg. CO₂

      ..           ..          ..            ..
   : O :: C :: O : or : O = C = O :
      ..           ..          ..            ..