Covalent Bonds and The Octet Rule

Noble gases have very stable valence shells.  When ions form, atoms tend to lose or gain electrons until a noble gas configuration is achieved.   This noble gas configuration is also responsible for the number of electrons an atom can share and the number of covalent bonds that will form.

Hydrogen can obtain a stable full shell with only 2 electrons. It is the exception rather than the rule.     H : H    The Lewis structure indicates that both atoms have access to the electrons in the bond.

The Octet Rule
The valence shells of all the noble gases, except helium, all contain eight electrons. There is the tendency for many atoms to acheive this noble gas configuration of 8 electrons. This is the basis of the octet ruleWhen atoms react, they tend to achieve an outer shell having eight electrons.   The octet rule can be used to explain the number of covalent bonds an atom forms. This number normally equals the number of electrons the atom needs to have a total of eight electrons.

In a chlorine atom, which has seven electrons in its valence shell,  we only need one electron to complete the octet.   Chlorine can of course gain total control of an electron and become a chloride ion.  This is what happens when it becomes an ionic compound.   When chlorine combines with another nonmetal, the transfer of electrons is not complete.  In compounds like HCl and Cl2  the chlorine gets the one electron it needs by sharing.

                      ..                        ..
       H .   +   . Cl : ------>   H : Cl :        OR    H - Cl
                     ..                         ..

Further examples:     Cl2                 CH4               NH3               H2O

These are examples of strucutral formulas that show how the atoms in a molecule are attached to each other.