THE BASICS OF DRAWING LEWIS STRUCTURES

Atoms in the main group are most stable when they have a complete valence shell of eight electrons. This is called the octet rule. They can do this by gaining or losing electrons to form ions, or by forming covalent bonds with shared pairs of electrons. Each electron can be represented by a dot. 

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Examples of ions include Na+, [Ca]2+, [Al]3+, [P]3-, [O]2-, Cl-.

Examples of atoms forming covalent bonds include Cl2 and P4. Each bond (line) represents two shared electrons.

Two atoms can also share four electrons to form a double bond, such as in O2, or six electrons to form a triple bond, such as in N2.

Electrons that are not used to form covalent bonds will pair up to form "lone pairs". Look again at Cl2, P4, O2 and N2. Lone pairs can have an effect on the shape of a molecule. Water, for example, has two lone pairs and this causes the bent shape of H2O.

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Note that H+, H-, He, Li+ and Mg2+ have a valence shell of 0 or 2 electrons.