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
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.
#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
#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.