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Section 3: Molecular Structures
Structural formulas show the relative positions of atoms within a
molecule.
K
What I Know
W
What I Want to Find Out
L
What I Learned
• 7(C) Construct electron dot formulas to illustrate ionic and
covalent bonds.
• 7(B) Write the chemical formulas of common polyatomic ions, ionic
compounds containing main group or transition metals, covalent
compounds, acids, and bases.
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Molecular Structures
Essential Questions
• What are the basic steps used to draw Lewis structures?
• Why does resonance occur, and what are some
resonance structures?
• Which molecules are exceptions to the octet rule, and why
do these exceptions occur?
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Molecular Structures
Vocabulary
Review
New
• ionic bond
• structural formula
• resonance
• coordinate covalent bond
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Molecular Structures
Structural Formulas
A structural formula uses letter symbols and bonds to show relative
positions of atoms.
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Molecular Structures
Structural Formulas
Drawing Lewis Structures
•
Predict the location of certain atoms. The atom that has the least
attraction for shared electrons will be the central atom in the molecule
(usually, the one closer to the left side of the periodic table). All other
atoms become terminal atoms. Note: Hydrogen is always a terminal
atom.
•
Determine the number of electrons available for bonding. The number
of valence electrons.
•
Determine the number of bonding pairs. Divide the number of electrons
available for bonding by two.
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Molecular Structures
Structural Formulas
•
Place the bonding pairs. Place a single bond between the central atoms
and each of the terminal atoms.
•
Determine the number of bonding pairs remaining. Subtract the number
of bonding pairs in step 4 from the number of bonding pairs in step 3.
Place lone pairs around terminal atoms, except hydrogen, to satisfy
the octet rule. Any remaining pairs will be assigned to the central atom.
•
Determine whether the central atom satisfies the octet rule. If not,
convert one or two of the lone pairs on the terminal atoms into a double
bond or a triple bond between the terminal atom and the central atom.
Remember: carbon, nitrogen, oxygen and sulfur often form double
and triple bonds.
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Molecular Structures
Structural Formulas
Atoms within a polyatomic ion are covalently bonded. The procedure for
drawing Lewis structures is similar to drawing them for covalent
compounds. The difference is, you need to determine the number of
electrons available for bonding. Find the number of electrons available in
the atoms present and then subtract the ion charge if the ion is positive or
add the ion charge if the ion is negative.
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Molecular Structures
LEWIS STRUCTURE FOR A COVALENT COMPOUND
WITH SINGLE BONDS
SOLVE FOR THE UNKNOWN
Use with Example Problem 3.
Problem
Find the total number of valence electrons
available for bonding.
5 valence electrons
+3 H atoms
1 N atom
1 valence electron
×
=8 valence electrons
1 H atom
1 N atom ×
Ammonia is a raw material used in the
manufacture of many products, including
fertilizers, cleaning products, and
explosives. Draw the Lewis structure for
ammonia (NH3).
There are 8 valence electrons available for
bonding.
Response
•
ANALYZE THE PROBLEM
Ammonia molecules consist of one
nitrogen atom and three hydrogen atoms.
Because hydrogen must be a terminal
atom, nitrogen is the central atom.
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Determine the total number of bonding
pairs. To do this, divide the number of
available electrons by two.
8 electrons
= 4 pairs
2 electrons/pair
Four pairs of electrons are available for
bonding.
Molecular Structures
LEWIS STRUCTURE FOR A COVALENT COMPOUND
WITH SINGLE BONDS
SOLVE FOR THE UNKNOWN
•
Place a bonding pair (a single bond)
between the central nitrogen atom and
each terminal hydrogen atom.
The remaining pair—a lone pair—must be
added to either the terminal atoms or the
central atom. Because hydrogen atoms
can have only one bond, they have no
lone pairs.
•
Place the remaining lone pair on the
central nitrogen atom.
Determine the number of bonding pairs
remaining.
•
Subtract the number of pairs used in
these bonds from the total number of
pairs of electrons available.
4 pairs total − 3 pairs used = 1 pair
available
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EVALUATE THE ANSWER
Each hydrogen atom shares one pair of
electrons, as required, and the central
nitrogen atom shares three pairs of
electrons and has one lone pair, providing
a stable octet.
Molecular Structures
LEWIS STRUCTURE FOR A COVALENT COMPOUND
WITH MULTIPLE BONDS
SOLVE FOR THE UNKNOWN
Use with Example Problem 4.
Problem
Carbon dioxide is a product of all cellular
respiration. Draw the Lewis structure for
carbon dioxide (CO2).
Response
ANALYZE THE PROBLEM
The carbon dioxide molecule consists of one
carbon atom and two oxygen atoms.
Because carbon has less attraction for
shared electrons, carbon is the central atom,
and the two oxygen atoms are terminal.
Find the total number of valence electrons
available for bonding.
4 valence electrons
1 C atom ×
+2 O
1 C atom
6 valence electrons
atoms ×
1 O atom
= 16 valence electrons
There are 16 valence electrons available
for bonding.
•
Determine the total number of bonding
pairs by dividing the number of
available electrons by two.
16 electrons
= 8 pairs
2 electrons/pair
Eight pairs of electrons are available for
bonding.
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Molecular Structures
LEWIS STRUCTURE FOR A COVALENT COMPOUND WITH
MULTIPLE BONDS
Determine the number of electron pairs
remaining.
SOLVE FOR THE UNKNOWN
•
•
Place a bonding pair (a single bond)
between the central carbon atom and
each terminal oxygen atom.
Subtract the lone pairs from the pairs
available.
6 pairs available − 6 pairs used = 0 pairs
available
Determine the number of electron pairs
remaining. Subtract the number of pairs used
in these bonds from the total number of pairs
of electrons available.
Examine the incomplete structure above
(showing the placement of the lone pairs).
Note that the carbon atom does not have an
octet and that there are no more electron
pairs available. To give the carbon atom an
octet, the molecule must form double bonds.
•
•
Subtract the number of pairs used in
these bonds from the total number of pairs
of electrons available.
Use a lone pair from each O atom to form
a double bond with the C atom.
8 pairs total − 2 pairs used = 6 pairs available
•
Add three lone pairs to each terminal
oxygen atom.
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EVALUATE THE ANSWER
Both carbon and oxygen now have an
octet, which satisfies the octet rule.
Molecular Structures
Lewis Structure for a Polyatomic Ion
SOLVE FOR THE UNKNOWN
Use with Example Problem 5.
Problem
Draw the correct Lewis structure for the
polyatomic ion phosphate (PO43-).
Response
ANALYZE THE PROBLEM
You are given that the phosphate ion consists
of one phosphorus atom and four oxygen
atoms and has a charge of 3-. Because
phosphorus has less attraction for shared
electrons than oxygen, phosphorus is the
central atom and the four oxygen atoms are
terminal atoms.
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Find the total number of valence electrons
available for bonding.
5 valence electrons
1 P atom ×
+4O
1 P atom
6 valence electrons
atoms ×
+ 3 electrons
1 O atom
from the negative charge = 32 valence
electrons
•
Determine the total number of bonding
pairs.
32 electrons
= 16 pairs
2 electrons/pair
•
Draw single bonds from each terminal
oxygen atom to the central phosphorus
atom.
Molecular Structures
Lewis Structure for a Polyatomic Ion
SOLVE FOR THE UNKNOWN
•
Subtract the number of pairs used from the
total number of pairs of electrons available.
EVALUATE THE ANSWER
All of the atoms have an octet, and
the group has a net charge of 3-.
16 pairs total − 4 pairs used = 12 pairs available
Add three lone pairs to each terminal oxygen
atom. 12 pairs available − 12 lone pairs used = 0.
•
Subtracting the lone pairs used from the pairs
available verifies that there are no electron
pairs available for the phosphorus atom. The
Lewis structure for the phosphate ion is
shown.
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Molecular Structures
Resonance Structures
Resonance is a condition that occurs when more than one valid Lewis
structure can be written for a molecule or ion. This figure shows three
correct ways to draw the structure for (NO3)-1.
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Molecular Structures
Resonance Structures
Two or more correct Lewis
structures that represent a single
ion or molecule are resonance
structures. The molecule behaves
as though it has only one
structure. The bond lengths are
identical to each other and
intermediate between single and
double covalent bonds.
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Molecular Structures
Exceptions to the Octet Rule
Some molecules do not obey the octet rule. A small group of molecules
might have an odd number of valence electrons. NO2 has five valence
electrons from nitrogen and 12 from oxygen and cannot form an exact
number of electron pairs.
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Molecular Structures
Exceptions to the Octet Rule
A few compounds form stable configurations with less than 8 electrons
around the atom—a suboctet. A coordinate covalent bond forms when
one atom donates both of the electrons to be shared with an atom or ion
that needs two electrons.
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Molecular Structures
Exceptions to the Octet Rule
A third group of compounds has central atoms with more than eight
valence electrons, called an expanded octet. Elements in period 3 or
higher have a d-orbital and can form more than four covalent bonds.
Copyright © McGraw-Hill Education
Molecular Structures
LEWIS STRUCTURE: EXCEPTION TO THE OCTET RULE
Use with Example Problem 6.
SOLVE FOR THE UNKNOWN
Problem
Response
First, find the total number of valence
electrons.
8 valence electrons
1 Xe atom ×
+4F
1 Xe atom
7 valence electrons
atoms ×
= 36
1 F atom
valence electrons
ANALYZE THE PROBLEM
•
Determine the total number of bonding
pairs.
36 electrons
=18 pairs
2 electrons/pair
•
Use four bonding pairs to bond the four
F atoms to the central Xe atom.
Xenon is a noble gas that will form a few
compounds with nonmetals that strongly
attract electrons. Draw the correct Lewis
structure for xenon tetrafluoride (XeF4).
You are given that a molecule of xenon
tetrafluoride consists of one xenon atom and
four fluorine atoms. Xenon has less attraction
for electrons, so it is the central atom.
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Molecular Structures
LEWIS STRUCTURE: EXCEPTION TO THE OCTET RULE
SOLVE FOR THE UNKNOWN
•
Determine the number of remaining
pairs.
18 pairs available - 4 pairs used = 14 pairs
available
•
EVALUATE THE ANSWER
This structure gives xenon 12 total
electrons, an expanded octet. Xenon
compounds, such as the XeF4 shown
here, are toxic because they are highly
reactive.
Add three pairs to each F atom to
obtain an octet. Determine how many
pairs remain.
3 pairs
14 pairs − 4 F atoms ×
1 F atom
= 2 pairs unused
•
Place the two remaining pairs on the
central Xe atom.
Copyright © McGraw-Hill Education
Molecular Structures
Review
Essential Questions
• What are the basic steps used to draw Lewis structures?
• Why does resonance occur, and what are some resonance
structures?
• Which molecules are exceptions to the octet rule, and why do these
exceptions occur?
Vocabulary
• structural formula
Copyright © McGraw-Hill Education
• resonance
• coordinate covalent
bond
Molecular Structures