Lewis Structures
The important bond in Organic Chemistry: the covalent bond
The covalent bond is the consequence of the sharing of the valence electrons between the atoms that form the bond
To know how bonds are established we will follow certain simple rules, stated by G.N. Lewis at the beginning of the 20th century.
Electrons around an atom are distributed in non-bonding pairs and shared pairs.
The sum of shared and unshared electrons around an atom must be 2, 8 or 18. The most common situation is eight: octet rule
To write the structure of a molecule (NH3 for example) the following rules must be followed:
1. The central atom is chosen (never hydrogen). Generally the least electronegative (which has the least "cost" to share electrons).
2. The valence electrons (outermost) of all atoms are counted (1 x N = 5, 3 x H = 3, total 8 electrons or 4 pairs).
3. Bonds are formed between the central atom and the peripheral ones.
4. The "excess" electrons are placed on the atoms as unshared pairs (never in hydrogen).
