Inert gas electron configurations (Lewis octets)
The following Periodic Table shows some of the elements
that appear most frequently in organic compounds. The numbers give
the number of valence electrons held by each atom:
Do you see any patterns? Atoms
in the same column have the same number of valence electrons.
Atoms in the same row have varying numbers of valence electrons,
but the number is always 8 or less. The inert gases (blue boxes)
always hold 8 valence electrons (He is the exception). 
Lewis postulated that inert gas atoms are chemically
inert because they hold 8 valence electrons. The valence electron
pattern in these atoms is called an inert gas electron configuration.
Chemists also call this a Lewis octet.
Lewis proposed that other atoms become more stable
(less chemically reactive) by adopting an inert gas electron configuration.
The atoms do this by ionizing (gaining or losing electrons) or by
sharing an electron pair with another atom.
Here are some familiar examples of "unstable"
atoms (left) and chemically stable ions or molecules (right):
has the same electron configuration as He
has the same electron configuration as Kr
- Each H in H2
"sees" two electrons, so each H has the same electron
configuration as He.
Questions #1-3 are based on the Periodic Table above.
I expect you to know these electron configurations without
having to consult the chart. Spend some time learning the chart
at the top of this page, and then see how much of the following
you can do.
#1. Which inert gas does not have a Lewis octet?
How many valence electrons does it have?
#2. Which atoms and/or ions can achieve inert
gas electron configurations without achieving Lewis octets?
#3. For each atom listed below give a) the
number of valence electrons it holds, b) the atomic ion that would
have an inert gas electron configuration: O, Br, P, S, K, F, Mg.
#4. Decide whether the atoms in these formulas
have inert gas electron configurations (note: a given formula might
contain a mix of atoms with and without proper configurations, therefore
you need to check every atom). Every time you find an atom that
lacks an inert gas electron configuration, say how many valence
electrons are configured around this atom.
#5. Same as #4.
 Since most inert gases
are chemically inert and their outer electrons are almost completely
unaffected by neighboring atoms, one might argue that inert gases
lack valence electrons. Thinking about things this way
does not change things - atoms still try to adopt
inert gas electron configurations - but it does make the phrase "Lewis
octet" look odd ("Lewis null" might make more sense).