1.4 Representation of Structure
Lewis structures represent the valence electrons of an atom as dots. By using Lewis structures one can recognize if any atoms possess lone-pair electrons or have a formal charge. For example, water and hydroxide ion can be represented by Lewis structures as shown below.
Valence electrons not used in bonding are called nonbonding electrons or lone-pair electrons. A positive or a negative charge assigned to an atom is called a formal charge; water molecule has no formal charge but the oxygen atom in the hydroxide ion has a formal charge of -1.
Nitrogen has five valence electrons and the Lewis structures of ammonia, ammonium ion, and amide anion are shown below. Ammonium ion has a formal charge of +1 and amide anion has a formal charge of -1.
Carbon has four valence electrons and the Lewis structures of methane, methyl cation, methyl anion, and methyl radical are shown below.
A species containing a positively charged carbon atom is called a carbocation , and a species containing a negatively charged carbon atom is called a carbanion . A species containing an atom with a single unpaired electron is called a radical or a free radical .
Hydrogen has one valence electron and each halogen (F, Cl, Br, I) has seven valence electrons, so the following species have the indicated formal charges:
A pair of shared electrons can also be shown as a line between two atoms. The hydrogen has one covalent bond and no lone pair. The halogens have one covalent bond and three lone pairs. The oxygen of water has two covalent bonds and two lone pairs. The nitrogen of ammonia has three covalent bonds and one lone pair. Each atom has a complete octet except hydrogen which has completely filled outer shell.
Kekulé structures represent the bonding electrons as lines and the lone-pair electrons are usually left out entirely, unless they are needed to draw a mechanism for a chemical reaction.
Sometimes some of the covalent bonds of the structure of a compound are omitted for simplification. These kinds of structures are called condensed structures.
