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  Lewis' Theory of Chemical Bonding Covalent Bonds Lewis' second great idea was this: two atoms attract each other (create a covalent bond) by sharing a pair of electrons. Lewis claimed that the shared electrons became part of each atom's electron configuration, so sharing effectively boosts each atom's electron count. For example, an isolated hydrogen atom possesses only one electron, but two hydrogen atoms can share their electrons so that the resulting covalent bond gives each atom an inert gas electron configuration:
In this case, electron sharing boosts boosts the number of electrons "seen" by each atom. Similarly, covalent bonding in the following compounds boosts each atom's electron count and gives the atom a Lewis octet. Each H in ammonia, NH3, "sees" the 2 electrons it shares with N. At the same time, the N "sees" 8 electrons; the 6 bonding electrons and the 2 nonbonding electrons it "keeps" for itself.
Two atoms can share more than one pair of electrons. The C and O in formaldehyde, CH2O, share 4 electrons. By forming a double bond, each atom achieves a Lewis octet.
Lewis' theory is numerically strict. A covalent bond involves two electrons, and it occurs between two atoms, and each atom "sees" both electrons to the same degree (the electrons are shared equally). These rules are powerful predictors of bonding behavior (especially the "bond = 2 electrons" rule), but we will eventually encounter molecules that disobey them.
Review problems #1. Describe each atom's electron configuration in the following molecule. (Give the total number of electrons "seen" by the atom, the number of bonding electrons, and the number of nonbonding electrons.)
#2. Same as #1.
#3. At sufficiently high temperatures, atom motion overwhelms the attractive force of the covalent bond, and molecules disintegrate into atoms. Suppose one takes a hot gas of H, F, and Cl atoms, and cools it so that molecules can form. What molecules might be expected from Lewis' theory? (Draw Lewis structures of these molecules if you can.)
#4. Which atoms in this formula have not achieved Lewis octets? Redraw this formula by changing nonbonding electrons into bonding electrons and giving each atom a Lewis octet.
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