Lewis Dot Diagrams & Structures
How are electrons shared to create covalently bonded molecules?
Read This!
Why? Covalent bonds result from a sharing of electrons by two or more atoms (usually
nonmetals). Lewis theory (Gilbert Newton Lewis, 1875-1946) focuses on the valence
electrons, since the outermost electrons are the ones that are highest in energy and farthest
from the nucleus, and are therefore the ones that are most exposed to other atoms when
bonds form. Lewis dot diagrams for elements are a handy way of picturing valence
electrons, and especially, what electrons are available to be shared in covalent bonds.
Model I: Valence Electrons / Lewis Dot Diagrams
H
He
Li
Be
B
C
N O
F
Ne
Na
Mg
Al
Si
P
Cl
Ar
S
Exploration I
1. How many valence electrons do the atoms H, Li and Na each have?
2. How many valence electrons do the atoms B and Al each have?
3. How many valence electrons do the atoms F and Cl each have?
4. Create a rule about how many valence electrons an atom will have.
5. Why do you believe that the electron dot diagram for the He atoms violates this rule?
Read This!
Notice that the placement of the electron dots is very purposeful. The first two (if applicable) are
always placed together. Then, any remaining valence electrons are placed individually on each of the
remaining three sides of the chemical symbol. After each side has one electron, then any remaining
valence electrons are paired on the three sides.
NOTE: On the Internet, OGT and some textbooks, the Lewis Dot Diagrams are not drawn
properly. For example, carbon is drawn:
C
6. What electrons are considered to be valence electrons: s, p, d, f? Explain how you came to this
conclusion.
7. Why are only these electrons considered valence electrons?
8. Hypothesize: Why are the valence electrons placed around an atom in the manner described?
Application: Use the atom cards and Cheerios to build the atom before drawing it.
9. Draw a Lewis dot diagram for the barium atom.
10. Draw the Lewis dot diagram for the silicon atom.
11. Draw the Lewis dot diagram for the iodine atom.
12. Draw the Lewis dot diagram for the xenon atom.
13. Hypothesize: Why are noble gases considered to be non-reactive?
Your group will check your answers with the instructor before moving on.
Model II: Lewis Dot Structures
Read This!
Covalent bonds generally form when a nonmetal combines with another nonmetal. Both elements in the
bond are attracted to the unpaired valence electrons so strongly that neither can take the electron away from
the other (unlike the case with ionic bonds), so the unpaired valence electrons are shared by the two atoms,
forming a covalent bond. The shared electrons act like they belong to both atoms in the bond, and they bind
the two atoms together into a molecule. The shared electrons are usually represented as a two dots (
)
or as a line (—) between the bonded atoms. These represent a single bond. (In Lewis structures, a line
represents two electrons.) Atoms tend to form covalent bonds in such a way as to satisfy the octet rule, with
every atom surrounded by eight electrons. (Hydrogen is an exception, since it is in period 1 of the periodic
table, and only has the 1s orbital available in the ground state, which can only hold two electrons.)
NOTE: A line (-) is used only for bonding electrons, never use it to represent a lone pair.
Example 1
H
F
H
=
F
H
F
Example 2
F
F
F
F
=
F
F
14. Identify the number of valence electrons in each atom of example 1.
15. How many total valence electrons are available to build the Lewis dot structure for HF?
16. Identify the number of valence electrons in each atom of example 2.
17. How many total valence electrons are available to build the Lewis dot structure for F2?
18. How many valence electrons does each H atom have?
19. How many total electrons are available to build the Lewis dot structure for H2?
20. How many valence electrons does each bromine atom have?
21. How many total electrons are available to build the Lewis dot structure for Br2?
Read This!
Use the atom cards and Cheerios to build the molecule before drawing it.
When building a Lewis dot structure for a molecule, it is important to first place one pair of electrons
between the atoms to be bonded. Once the atoms have been bonded, then valence electrons are place in pairs
around one atom until it has an octet. Then any remaining valence electrons are placed around another atom
until it also has an octet. NEVER: place individual electrons! All unbonded electrons are called lone pair.
Extension Questions
1.
Draw a Lewis dot structure for the H2 molecule.
2. Draw the Lewis dot structure for the Br2 molecule.
3. Draw the Lewis dot structure for the HI molecule.
Your group will check your answers with the instructor before moving on.
Read This!
When building molecules with more than two atoms, the structure will begin with one central atom, placed
in the middle, with up to four surrounding atoms, placed on the top, bottom, right and left sides of the
central atom. Deciding which atom is the central atom? NEVER use hydrogen or a halogen! They typically
only form one bond as they achieve an octet. The central atom should be the atom that is farthest from the
fluorine atom on the periodic table.
Rules for building Lewis dot structures:
1. Count the total number of valence electrons in each atom and add them together to get the total
number of electrons in the molecule. (For example, H2O has 21 + 6 = 8 valence electrons, CCl4 has
4 + 47 = 32 valence electrons.)
2. Place the atoms relative to each other. Place the atom that is farthest from fluorine on the periodic
table, as the central atom. Place the remaining atoms on the top, bottom, left and ride sides.
Remember: Hydrogen and Halogens are never central atoms.
3. Place a pair of electrons between each atom to form a bond. Each bond uses two valence electrons.
4. Complete an octet on one surrounding atom before proceeding to the next surrounding atom. Once
all surrounding atoms have octets, any remaining electrons should be placed on the central atom to
achieve an octet. The number of electrons in the final structure must equal the number of
valence electrons from Step 1.
Read This!
Lewis dot structure for CH4
Step 1: C has 4 valence electrons; each H has 1 valence electron. Total electrons = 8
Step 2: Place C as the central atom
H
H
C
H
H
Step 3: Place electrons to form bonds
H
H -
C -
H
H
Step 4: There were 8 valence electrons in step 1 and 8 electrons in the Lewis dot structure.
Application:
Use the atom cards and Cheerios to build the molecule before drawing it.
1. Complete the Lewis dot Structure for CCl4
2. Complete the Lewis dot structure for NH3
3. Complete the Lewis dot structure for H2O
Model III: Complex Lewis Dot Structures / Polyatomic Ions
Read This!
Use the atom cards and Cheerios to build the polyatomic ion before drawing it.
Polyatomic ions are a group of atoms that are covalently bonded and act as a single unit with a charge.
Building Lewis dot structures for polyatomic ions follows the same process as building a molecule. So what is
the difference? The charge! A 2+ charge means that the Lewis dot structure will have 2 FEWER electrons in
the structure than what is calculated in step 1. A 3- charge means that the Lewis dot structure will have 3
MORE electrons than what is calculate in step 1. Once the structure is completed, a bracket [ ]charge will be
placed around the entire structure.
4. Complete the Lewis dot structure for H3O+
5. Complete the Lewis dot structure for OH-
6. Complete the Lewis dot structure for NH4+
7. Complete the Lewis dot structure for SO42-
Your group will check your answers with the instructor before moving on.
Model IV: Double and Triple Bonds
Read This!
Use the atom cards and Cheerios to build the molecule before drawing it.
When following the four rules for building a Lewis dot structure, it may appear as though there are not
enough valence electrons to complete the structure. If this happens, you will move a lone pair from the one
of the surrounding atoms, and place it in between that atom and the central atom. This will create a double
bond. If this doesn’t result in all atoms achieving an octet, move another lone pair from a surrounding atom
and place it between that atom and the central atom. This sharing of three pair of electrons is called a triple
bond. Atoms never share more than three pair of electrons! WARNING: Do not create double or triple
bonds unless it is needed. Hydrogen and Halogens never have double or triple bonds.
Building O2: Total valence electrons = 12
Following steps 1-4:
O - O
Move one lone pair:
O - O
to produce
O = O
Total electrons = 12
8. Complete the Lewis dot structure for N2
9. Complete the Lewis dot structure for CO2
10. Complete the Lewis dot structure for CO
11. Complete the Lewis dot structure for HCN
Your group will check your answers with the instructor before moving on.
Model V: Resonance Structures— When One Lewis Structure Isn’t Enough
Read This!
Step 1: Build a Lewis dot structure for ozone, O3
Total valence electrons (36) = 18
Steps 2 and 3: Place one O in the center, and connect the other two O’s to it. Drawing a single bond from
the central atom to each of the surrounding atoms.
O - O- O
Step 4: Place electrons around surrounding atoms first to complete octets. Place the remaining two electrons
on the central atom.
We can satisfy the octet rule on the central O by making a double bond either between the left O and the
central one or the right O and the center one:
O = O - O
or
O - O = O
The question is, which one is the “correct” Lewis structure? In reality, neither one is correct, due to the fact
that the electrons are in constant motion. The bond lengths of each bond are equal to each other;
approximately a 1 ½ bond. This is where the typical model fails, so we draw all possible structures with a
double sided arrow between them. These are called resonance structures.
Application: Use the atom cards and Cheerios to build the molecule before drawing it.
1. Complete the Lewis dot structure(s) for NO3-
2. Complete the Lewis dot structure(s) for CO32-
3. Complete the Lewis dot structure(s) for COCl2
4. Complete the Lewis dot structure(s) for NO2-
5. Complete the Lewis dot structure(s) for ClO3-
Your group will check your answers with the instructor before moving on.
Model VI: Multi-Centered Molecules
Read This!
Molecules with more than one central atom are drawn similarly to the ones above. The octet rule
can be used as a guideline in many cases to decide in which order to connect atoms.
C2H6 Total valence electrons = 14
Each carbon atom will represent a central atom and they will be bonded to each other. The
remaining hydrogen atoms will be placed symmetrically around each central atom. Be sure your final
structure has the correct number of electrons.
H
H
-
C
H
H
-
C
H
-
H
Total electrons = 14
Application:
Use the atom cards and Cheerios to build the molecule before drawing it.
1. Complete the Lewis dot structure(s) for C2H4
2. Complete the Lewis dot structure(s) for C2H2
3. Complete the Lewis dot structure(s) for CH3OH (Hint: The three H atoms are connected to
the C atom and the last H atom is connected to the O atom)
4. Complete the Lewis dot structure(s) for NH2OH
5. Complete the Lewis dot structure(s) for N2F6
6. Complete the Lewis dot structure(s) for N2H4