Electron Dot Diagrams / Lewis
Structures
Atom and Covalent Compound
diagramming
Electron Dot Diagrams
• Shows valence electrons around an atom
• Valence electrons – electrons in the outermost
energy level of an atom
• Valence electrons are only located in the outer level
“s” and “p” orbitals
• Valence electrons are shown in the outer energy
level of a “shell diagram”
Electron Shell Diagrams
• A shell diagram shows
all electrons, but we
are most interested in
the electrons in the
outermost energy level
 called the valence
level or valence shell.
Electron Dot Diagrams
•
To draw:
1. Write chemical
symbol
2. Place one dot around
chemical symbol for each
valence electron (up to
eight dots)
3. All inner level electrons and
nucleus are symbolized
by the chemical symbol
Electron Dot Diagrams
• Draw electron dot diagrams of the following
elements:
1. Carbon
6. Hydrogen
2. Nitrogen 7. Chlorine
3. Phosphorus 8. Argon
4. Boron
9. Iodine
5. Oxygen
10. Helium
Electron Dot Diagrams
• Line up single dots to
form compounds and
to make either 2 or 8
electrons around each
element. The
elements share their
single unpaired
electrons.
The Basics: Drawing Lewis Structures
Step 1: Calculate the total number of valence electrons in the
molecule or ion
Step 2: Determine the central atom(s) of the molecule or ion –
usually it’s the least electronegative atom.
Step 3: Draw a tentative diagram for the molecule or ion.
Rules
a) A hydrogen atom always forms one bond. Hydrogen
is always a terminal atom in a Lewis diagram – an
atom that is bonded to only one other atom.
b) A carbon atom normally forms four bonds
c) When several carbon atoms appear in the same
molecule, the are often bonded to each other. (No
cyclic compounds in Chem 60/68)
Electron Dot Diagrams
• Some electrons are already paired up within a
single atom and are not shared. These are called
non-bonding / unshared pairs.
Electron Dot Diagrams
• Some elements
are able to share
more than one
pair of electrons
to form double
and triple bonds
Draw the Lewis Structure for the following molecules.
1. H2O
Oxygen has 6 valence electrons & Hydrogen has 1 valence electron for a
total of 8 electrons.
..
H:O:H
..
2. CO
Oxygen has 6 valence electrons & Carbon has 4 valence electrons for a total
of 10 electrons.
:C:::O:
Draw the Lewis Structure for the following molecules.
3. BH3
Boron has 3 valence electrons & Hydrogen has 1 valence electron for a total
of 6 electrons
H:B:H
..
H
4. NH3
Nitrogen has 5 valence electrons & Hydrogen has 1 valence electron for a
total of 8 electrons
..
H:N:H
..
H
p. 362
Cations
p. 363
Anions
p. 366
Resonance
p. 366
Homework
• Draw electron dot diagrams of the following
compounds:
1. CCl4
2. N2O2
3. PBr3
4. BI3
5. C3H6O
6. H2O
7. CH4
8. SO2
9. NH3
10. SiO4