Lewis Structures
A way to represent bonding in a molecule
PCl3 How to draw?
1) Atom with lower group number (P) needs more electrons to complete the valence
shell. Place this atom in the center and locate other atoms around it.
Cl
P
Cl
Cl
H will never be a central atom since it can only form one bond (it can accommodate only
2 valence electrons)
2) Determine the total number of valence electrons:
Phosphorous has 5 valence electrons and each Chlorine has 7 electrons
5 + 7(3) = 26 total valence electrons in PCl3
Note: For a positively charged species, subtract one electron per charge unit (i.e. for a
mono-cation, subtract one electron, for a dication, subtract two electrons, etc.)
+
+
NH4 5 + 4 = 9 valence electrons but since it is a cation, NH4 will only have 8 valence
electrons.
For a negatively charged species, add one electron per charge unit.
-
OCH3 6 + 4 + 3 =13 electrons + 1 electron (due to negative charge) = 14 valence
electrons in the methoxide anion.
Back to the PCl3 example:
3) Draw a single bond between the central atom and the outer atoms
Cl
P
Cl
Cl
In this case, 6 electrons are used up (2 for each bond). This leaves 20 electrons unused.
4) Put the remaining electrons on atoms as lone pairs to complete the valence shell for
each atom.
P has 6 electrons (needs 2) and each Cl needs 6 electrons. This totals 20 electrons. So
all the remaining electrons get used up.
:
: Cl :
: P
:
:
Cl :
: Cl
:
:
For multiply bonded species:
Follow same initial steps as before
CO2
1)
O
C
O
2(6) + 4 = 16 electrons
O
C
O
2)
4 electrons have been used. 12 electrons remain. Each oxygen needs 6 electrons to
complete its valence shell and the carbon needs 4 electrons to complete its valence shell.
3) Place lone pairs around atoms:
One way to do it....
:
:
:O
C
O:
:
:
The 12 remaining electrons have been placed around the atoms. Each oxygen atom now
has a complete valence shell, but the carbon is still electron deficient.
4) Convert a lone pair to a bonding pair:
:O
:
C
O:
C
:
:
O:
:
:
:
:
:O
Now there is a double bond between the carbon and one of the oxygens, but the carbon is
still electron deficient. Repeat the process using a lone pair from the other oxygen.
:
:
:
Now all atoms are happy.
O:
:O
C
O:
:
C
:
:O