Transcript 9_3 Molecular Structurex

Section 9.3
1
List the basic steps used to draw Lewis structures.
Explain why resonance occurs, and identify
resonance structures.
Identify three exceptions to the octet rule, and
name molecules in which these exceptions occur.
ionic bond: the electrostatic force that holds oppositely
charged particles together in an ionic compound
Resonance
structural formula
resonance
coordinate covalent bond
Write a paragraph that includes all of the
terms above.
Section 9.3 Molecular Structures (cont.)
Section 9-3
Structural formulas show the relative
positions of atoms within a molecule.
A structural formula uses letter symbols and bonds to
show relative positions of atoms.
Drawing Lewis Structures
– Predict the location of certain atoms.
– Determine the number of electrons
available for bonding.
– Determine the number of bonding pairs.
– Place the bonding pairs.
– Determine the number of bonding pairs
remaining.
– Determine whether the central atom
satisfies the octet rule.
Atoms within a polyatomic ion are covalently bonded.
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.
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.
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.
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.
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.
Molecular Structures
Mastering Concepts: 272(79)
Terms: 252
Practice Problems: 255(30-34), 256(35-38),
258(39-41)
Cornell Notes: 9.3
Section Assessment: 258(42-46)
Mastering Problems: 272(99-104)
15
Mastering Concepts: 272(79)
79. What must be known in order to draw
the Lewis structure for a molecule? (9.3)
the number of valence electrons for each
atom
Molecular Structures
Homework:
Mastering Concepts: 272(79)
Terms: 252
Practice Problems: 255(30-34), 256(35-38),
258(39-41)
Homework:
Cornell Notes: 9.3
Section Assessment: 258(42-46)
Mastering Problems: 272(99-104)
17
Practice Problems: 255 (30-34)
Draw a Lewis structure for each of the following:
30. NF3
31. CS2
32. BH3
33. ClO434. NH4+
18
Compound
30. NF3
1) total
valence
electrons 5+ 3(7) = 26
3) Electron
pairs in
bonds
26 – 6= 20
4) Octet for
outer atoms
..
:F:
N:
:F:
..
2) Skeletal
Structure
:F:
..
F
N
F
F
19
Compound
4) Octet for
outer atoms
..
31. CS2
:S
..
1) total
4+ 2(6) = 5) Remaining ..
:S
valence
16
e– s on
electrons
center atom ..
2) Skeletal
Structure
3) Electron
pairs in
bonds
S C
S
16-4=12
6) Create
multiple
bonds?
..
:S
C
..
S:
..
C
..
S:
..
..
C S:
Final
structure
21
Compound
4) Octet for
outer atoms
32. BH3
1) total
valence
electrons
3+ 3(1) = 5) Remaining
6
e– s on
center atom
2) Skeletal
Structure
H
6) Create
multiple
bonds?
B
H
3) Electron
pairs in
bonds
none
H
6-6=0
Final
structure
23
Compound
4) Octet for
outer atoms
..
:O
..
33. ClO4-
..
O:
..
Cl
..
:O
..
O:
..
..
1) total valence
electrons
2) Skeletal
Structure
5) Remaining e–
s on center
atom
O
6) Create
multiple bonds?
O
Cl
O
3) Electron
7+4(6)+1 =
32
O
Final structure
25
Compound
3) Electron
pairs in
bonds
34. NH4+
8 – 8=0
1) total
4) Octet for
valence 5+4 (1) -1= outer atoms
electrons
8
2) Skeletal
Structure
H
H
N
H
H
27
Resonance structures
• For example, the bond lengths of CHO2–
predicted by the Lewis structure are incorrect
[
O
H
C
O
]
–
• The double CO bond should be shorter, and
possess a greater bond energy (due to the higher
concentration of e–s in a double bond)
• Yet, experimentally, both bonds are the same
• The reason is due to “resonance”
Resonance structures
C
O
O
H
C
O
[
] [
H
]
–
C
O
2
[
H
–
O
1
O
]
–
Resonance structures
• A resonance structure can be drawn for any molecule
in which a double bond can be formed from two or
more identical choices
• Resonance structures can be drawn 2 ways…
C
O
O
H
C
O
[
] [
H
]
–
C
O
2
[
H
–
O
1
O
]
–
Molecular Structures
Mastering Concepts: 272(79)
Terms: 252
Practice Problems: 255(30-34), 256(35-38),
258(39-41)
Cornell Notes: 9.3
Section Assessment: 258(42-46)
Mastering Problems: 272(99-104)
32
Practice Problems: 256 (35-38)
Draw the Lewis resonance structures for the
following.
35. SO3
36. SO2
37. O3
38. NO233
Compound
35. SO3
..
:O:
4) Octet for
outer
atoms
S
:O:
:O:
..
1) total valence
electrons
2) Skeletal
Structure
..
6+ (3)6 = 5) Remaining e–
s on center atom
24
O
S
O
O
24 -6=18
6) Create
multiple
bonds?
:O:
S
:O:
..
:O:
..
34
4) Octet for ..
:O
outer
..
atoms
S
..
O:
6+ (2)6 = 5) Remaining e– ..
s on center atom :O
18
..
..
S
..
O:
6) Create ..
multiple :O
..
O bonds?
..
S
..
O:
Compound
36. SO2
1) total valence
electrons
2) Skeletal
Structure
O S
18 -4=14
..
..
36
Compound
37. O3
1) total valence (3)6 = 18
electrons
4) Octet for
outer
atoms :O:
..
O
5) Remaining e–
..
O
s on center atom
..
:O
:O:
..
..
O:
..
2) Skeletal
Structure
O
O
18 -4=14
6) Create
multiple ..
O bonds? :O
..
..
O
..
O:
..
38
Compound
38.
NO2-
4) Octet for
outer
atoms :O:
..
1) total valance 5+(2)6+1 = 5) Remaining e–
electrons
s on center atom ..
18
2) Skeletal
Structure
N
O
18 -4=14
N
:O:
..
..
N
:O
..
O:
..
..
6) Create
multiple ..
O bonds? :O
..
N ..
O:
..
40
Molecular Structures
Mastering Concepts: 272(79)
Terms: 252
Practice Problems: 255(30-34), 256(35-38),
258(39-41)
Cornell Notes: 9.3
Section Assessment: 258(42-46)
Mastering Problems: 272(99-104)
42
Practice Problems: 258 (39-41)
Draw the correct Lewis structures for the
following molecules, which contain expanded
octet.
39. SF6
40. PCl5
41. ClF3
43
Practice Problems: 258 (39-41)
Draw the correct Lewis structures for the
following molecules, which contain expanded
octet.
39. SF6
44
Practice Problems: 258 (39-41)
Draw the correct Lewis structures for the
following molecules, which contain expanded
octet.
40. PCl5
46
Practice Problems: 258 (39-41)
Draw the correct Lewis structures for the
following molecules, which contain expanded
octet.
41. ClF3
48
Practice Quiz 975 (4-7)
Section 9-3
50