Chapter 13
Lewis Structures
Lewis Structures
• Lewis structures are diagrams that show
the bonding between atoms of a molecule,
and the lone pairs of valence electrons in
the molecule.
Steps for Writing Lewis Structures
1. Count the total number of outer level
(valence) electrons. To do this use the
Roman numeral from the group number
above each element in the periodic table.
• PO4
• PO43• PO43+
Steps for Writing Lewis Structures
2.
Determine the layout of the molecule. The formula of the
molecule will often give you a hint as to its layout. For
example in the molecule H3CCH3 there are two carbon atoms
in the center with three hydrogen atoms bonded to each
carbon.
Carbon is always central
Hydrogen is never central
The element with the lowest electronegativity is central
The element which you have the least of is usually central
Group 17 elements are usually not central (unless you have
no other choice) because they can only form single bonds.
•
•
•
•
•
–
After determining the layout of the molecule. Arrange the elements
symmetrically around your central atom(s).
Steps for Writing Lewis Structures
3. The valence electrons from step 1 are now used to
stabilize the atoms. This is done by using shared pairs
(bonds) (see Table 1) to attach the atoms to the central
atom. Use single bonds for all atoms first.
Valence electrons in Lewis Structures Appear as:
(1) Shared Pairs (form bonds)
—
Single bond (one shared pair)
Weakest and longest covalent bond
═
Double bond (two shared pairs)
Strength and length between single and triple
bonds
≡
Triple bond (three shared pairs)
Strongest and shortest covalent bond
(2) Unshared Pairs (Lone
Pairs)
xx, ○○, or ●●
Steps for Writing Lewis Structures
3. After adding single bonds you may find that the atoms
still need more valence electrons to achieve their
octets. If more valence electrons are needed use
unshared pairs (lone pairs) (see Table 1) around each
atom to give the atom an octet (8 valence electrons).
There are some exceptions to the octet rule. The most
common exception is hydrogen which only requires 2
electrons to fill its outer level and become stable.
Valence electrons in Lewis Structures Appear as:
(1) Shared Pairs (form bonds)
—
Single bond (one shared pair)
Weakest and longest covalent bond
═
Double bond (two shared pairs)
Strength and length between single and triple
bonds
≡
Triple bond (three shared pairs)
Strongest and shortest covalent bond
(2) Unshared Pairs (Lone
Pairs)
xx, ○○, or ●●
Steps for Writing Lewis Structures
3. The valence electrons from step 1 are now used to
stabilize the atoms. This is done by using shared pairs
(bonds) (see Table 1) to attach the atoms to the central
atom. Use single bonds for all atoms first.
• After adding single bonds you may find that the atoms
still need more valence electrons to achieve their octets.
If more valence electrons are needed use unshared
pairs (lone pairs) (see Table 1) around each atom to
give the atom an octet (8 valence electrons). There are
some exceptions to the octet rule. The most common
exception is hydrogen which only requires 2 electrons to
fill its outer level and become stable.
• If you still do not have a stable structure you may try
double and triple bonds if C, N, or O is involved in
the bond.
Steps for Writing Lewis Structures
4. If you cannot write a stable structure for the
molecule using rules 1 – 3 add or remove
unshared pairs to/from the central atom until
you arrive at the desired number of valence
electrons determined in step 1. This may give
you a structure that appears to be unstable
however some molecules can form which do
not have stable octets.
• Common exceptions to the octet rule: H is
stable with 2 valence electrons; B is stable with
6 valence electrons.
Writing Lewis Structures
CO2
Writing Lewis Structures
PO4
3-
Writing Lewis Structures
H2CO
Writing Lewis Structures
BrNO
Writing Lewis Structures
BrNO
Steps for Writing Lewis Structures
2.
Determine the layout of the molecule. The formula of the
molecule will often give you a hint as to its layout. For
example in the molecule H3CCH3 there are two carbon atoms
in the center with three hydrogen atoms bonded to each
carbon.
Carbon is always central
Hydrogen is never central
The element with the lowest electronegativity is central
The element which you have the least of is usually central
Group 17 elements are usually not central (unless you have
no other choice) because they can only form single bonds.
•
•
•
•
•
–
After determining the layout of the molecule. Arrange the elements
symmetrically around your central atom(s).
Writing Lewis Structures
HCCH
Steps for Writing Lewis Structures
2.
Determine the layout of the molecule. The formula of the
molecule will often give you a hint as to its layout. For
example in the molecule H3CCH3 there are two carbon atoms
in the center with three hydrogen atoms bonded to each
carbon.
Carbon is always central
Hydrogen is never central
The element with the lowest electronegativity is central
The element which you have the least of is usually central
Group 17 elements are usually not central (unless you have
no other choice) because they can only form single bonds.
•
•
•
•
•
–
After determining the layout of the molecule. Arrange the elements
symmetrically around your central atom(s).
Writing Lewis Structures
HCCH
Writing Lewis Structures
HCCH
Writing Lewis Structures
BeCl2
Steps for Writing Lewis Structures
4. If you cannot write a stable structure for the
molecule using rules 1 – 3 add or remove
unshared pairs to/from the central atom until
you arrive at the desired number of valence
electrons determined in step 1. This may give
you a structure that appears to be unstable
however some molecules can form which do
not have stable octets.
Homework
 Lewis Structures Worksheet
Homework
 Lewis Structures Worksheet