LEWIS STRUCTURES CLIFF NOTES
Name:________________
The goal of the Lewis structure is to have all atoms get an octet after the structure is complete!
(H only wants 2 e- and B wants 6)
1. Write the chemical formula, and count the total # of valence electrons.
This is the amount of electrons allowed in the Lewis Structure
These electrons can be placed anywhere in the structure and don’t belong to any specific atom
2. Arrange the atoms around each other.
Hints:
a) The least electronegative atom typically goes in the center - usually C
b) Hydrogen is terminal (at the end b/c it only can form 1 bond)
3. Put in electrons (dots) around the atoms to satisfy the octet of all the atoms
-
-
4. Check to see that you have the same # e s as in step 1, if not, rearrange the # e s and/or atoms.
5. CONNECT THE DOTS between atoms. 2 e- = a bond, denoted by a line ( : = -)
Guided practice problems:
carbon monoxide
CH2O
Lewis Structures Practice Worksheet 1
Draw the Lewis structures for the following:
a)
Nitrogen trifluoride
b)
Methane
c)
Fluorine
d)
oxygen
e)
Water
f)
Sulfur dichloride
g) Phosphorus pentachloride
h) Methyl Alcohol (CH3OH)
More detailed information
DRAWING LEWIS STRUCTURES
Start by learning this basic step-by-step procedure for drawing Lewis structures.
1. Draw a skeleton structure. A skeleton structure is a rough map showing the arrangement of
atoms within the molecule. In general, you need to determine the skeleton experimentally, but
here are a few guidelines for predicting skeleton structures from molecular formulas.
o
Central atoms are usually

the largest atoms, or

the least electronegative atom.
o
H and the halogens are usually outside atoms.
o
Don't put more than four atoms around a central atom unless the central atom is third
period or lower.
2. Count total valence electrons.
o
Add the number of electrons in the valence shells of all atoms in the molecule.
o
If the molecule is charged, add an electron for each negative charge and subtract an
electron for each positive charge.
o
Noble gas compounds are very uncommon (except on general chemistry tests!)
Should you encounter one, each noble gas atom has 8 valence electrons.
3. Connect the structure.
o
Draw a bond between the central atom and each outside atom.
o
Each bond uses 2 valence electrons.
4. Place electrons on outside atoms.
o
Use remaining electrons to satisfy the octets for each of the outside atoms.
o
If you run out of electrons at this point, the skeleton structure was wrong. Go back to
step I.
5. Place all remaining electrons on the central atom.
o
If there are more than 8 electrons on the central atom, and the central atom is not
third period or lower, you counted the number of valence electrons incorrectly. Go
back to step II.
o
If the octet on the central atom is not complete, try sharing lone pairs of outside
atoms to form double or triple bonds. Write one multiply bound structure for each
outside atom with a lone pair to share; these are resonance structures.
o
If you can't get an octet on the central atom, at this point, check to see whether the
total number of valence electrons for this molecule is odd. It's impossible to give
octets to all atoms in an odd electron molecules. Get as close to an octet as possible
by forming multiple bonds.
Author: Fred Senese senese@antoine.frostburg.edu
Lewis Structures Practice Worksheet 2
Draw the best Lewis structures for the following molecules or ions:
a) Phosphorus tribromide
f) oxygen
b) (Diazine) Dinitrogen dihydride
g) nitrogen
c) Methanol CH3OH
h) formaldehyde CH2O
d) Nitrite ion
i) nitrite ion (draw differently than letter d)
e) Ethene C2H4
j) Acetylene C2H2
Lewis Structures Practice Worksheet 3
Draw the best Lewis structures for the following molecules or ions:
k) SBF (keep boron central)
p) bromine
l) HBr
q) iodine
m) C2H5OH (ethanol)
r) fluorine
n) dinitrogen tetrafluoride
s) What do you notice about the formulas of the
elements of hydrogen, nitrogen, oxygen, fluorine,
chlorine, bromine and iodine?
o) sulfur hexafluoride
t) carbon dioxide
Resonance Structures Practice
Draw all of the possible resonance structures for the following ions or molecules:
1)
nitrate ion:
2)
formate ion (CHO2 ):
3)
cyclobutadiene (C4H4):
4)
ozone (O3):
-1