Organic Chemistry Workshop
Main Elements
● Carbon – C
● Hydrogen – H
● Nitrogen – N
● Oxygen – O
● Phosphorus – P
● Sulfur - S
Valence Electrons
Drawing Lewis structures
● 1) Count the total valence electrons for the
molecule: To do this, find the number of valence
electrons for each atom in the molecule, and add them up.
● 2) Figure out how many octet electrons the molecule
should have, using the octet rule: The octet rule tells us
that all atoms want eight valence electrons (except for
hydrogen, which wants only two), so they can be like the
nearest noble gas. Use the octet rule to figure out how
many electrons each atom in the molecule should have,
and add them up. The only weird element is boron - it
wants six electrons.
Drawing Lewis structures
● 3) Subtract the valence electrons from octet
electrons: Or, in other words, subtract the
number you found in #1 above from the number
you found in #2 above. The answer you get will
be equal to the number of bonding electrons in
the molecule.
● 4) Divide the number of bonding electrons
by two: Remember, because every bond has two
electrons, the number of bonds in the molecule
will be equal to the number of bonding electrons
divided by two.
Drawing Lewis structures
● 5) Draw an arrangement of the atoms for the
molecule that contains the number of bonds
you found in #4 above: Some handy rules to
remember are these:
○ Hydrogen and the halogens bond once.
○ The family oxygen is in bonds twice.
○ The family nitrogen is in bonds three times. So does
boron.
○ The family carbon is in bonds four times.
● A good thing to do is to bond all the atoms
together by single bonds, and then add the
multiple bonds until the rules above are followed.
Drawing Lewis structures
● 6) Find the number of lone pair
(nonbonding) electrons by subtracting
the bonding electrons (#3 above) from
the valence electrons (#1 above).
Arrange these around the atoms until all
of them satisfy the octet rule:
Remember, ALL elements EXCEPT
hydrogen want eight electrons around
them, total. Hydrogen only wants two
electrons.
Example
Formal Charge
● A concept know as formal charge can help us
choose the most plausible Lewis structure
where there are a number of structures which
would all be satisfactory according to the
rules used thus far. For example if we look at
the cyanate ion, NCO-, we see that it is
possible to write for the skeletal structure,
NOC-, CNO-, or CON-. Using formal charge we
can choose the most plausible of these three
Lewis structures.
Formal Charge
● To determine the formal charge of an
atom we first assign valence electrons
to atoms in a Lewis structure as follows.
● All unshared (lone pair) electrons are
assigned to the atom on which they are
found.
● Half of the electrons in a bond between
two atoms are assigned to one atom and
half to the other.
● After assigning all the valence electrons,
we determine the formal charges as
follows. The formal charge on an atom
is the number of valence electrons on
the free (nonbonded) atom minus the
number of electrons assigned to that
atom in the Lewis structure. We can
state this mathematically as follows.
● FC = V - (L + 1/2 S)
● Where V is the number of valence electrons
on the free atom, L is the number of electrons
present as lone pairs and S is the number of
shared electrons. Note that the sum of the
formal charges on all the atoms in a molecule
will be zero while for an ion it will be equal to
the charge on the ion. In using the concept of
formal charge we should keep the following
rules in mind.
● Usually the most plausible Lewis structure is one with
no formal charges (formal charges of zero on all
atoms).
● Where formal charges are required, they should be as
small as possible, and negative formal charges
should appear on the most electronegative atoms.
● Adjacent atoms in a structure should not carry formal
charges of the same sign.
● Formal charges on the atoms in a Lewis structure
must total to zero for a molecule and to the net charge
for a polyatomic ion.
● Getting back to the example of the
cyanate ion, the three Lewis structures
with the formal charges indicated are
shown below. Structure (a) would be
the most plausible Lewis structure.
● a bc