Ppt23 (PS11)
• (Prior Ppt with procedure to generate one “good” LDS
reflects Sections 9.5 and 9.7 in Tro. I will defer the
discussion on electronegativity and bond polarity [9.6 in
Tro] until later)
• Bond energies and Bond Lengths (9.10 in Tro)
• Resonance Structures/Forms (9.8 in Tro)
• Formal Charges—Concept and Application
(9.8 and part of 9.9 [expanded octets] in Tro)
• Special Case scenarios—Simple Organic
Compounds
Ppt23(PS11)
1
Practice (if time, need): Draw
Lewis Structures For the Following
•
•
•
•
•
•
•
Br3O3
CO2
PO43SO42ClF3
HCN
NOTE: Basic LDS’s are
covered in Section 9.5 and 9.7
in Tro. We will be continuing
with ideas in Sections 9.7 - 9.10
in this PowerPoint, although I
will discuss 9.10 (bond energies &
lengths) first. I’ll discuss Section
9.6 (electronegativity and bond polarity) in the
next PowerPoint presentation.)
Ppt23(PS11)
2
Like Fig. 10.6 in Tro. A bond is a state of lowered energy. A covalent bond
is a shared pair of electrons—energy is lowered if electrons get to “see” (be
attracted to) two nuclei instead of one. Bond energy is a measure of how
hard it is to break a given bond. (E is released if a bond is made!)
nuclei repel (energetically unfavorable)
DH-H (bond energy of H-H) = +436 kJ/mol
Ppt23(PS11)
3
Bond Energy
• The energy required to break apart a mole
of Cl2 molecules into Cl atoms is 243 kJ:
Cl2(g)  2 Cl(g) ; DH = 243 kJ
Thus, 243 kJ/mol is the bond energy of Cl-Cl
NOTE: The larger the bond energy:
the stronger the bond and
the lower the bond “is” in potential energy.
Strong bonds don’t “have” a lot of energy!
They’ve lowered themselves a lot in PE.
Ppt23(PS11)
4
Ppt23(PS11)
5
When one atom is the same (e.g., H here), the bond lengths
trend as the atomic radius of the other atom.
Ppt23(PS11)
6
For bonds between the same two atoms, as bond
order (1 = single; 2 = double, 3 = triple)* increases,
bonds get stronger and shorter.
Bond Energy
(kJ/mol)
347
611
837
305
615
891
360
736
163
418
946
Ppt23(PS11)
222
590
*As we shall soon see,
non-integer bond orders
(e.g., 4/3, 1.5) are also
possible.
7
Lewis Dot Structures-II
• Resonance Structures/Forms
• Formal Charge—Concept and Application
• Special Case scenarios—Simple Organic
Compounds
Ppt23(PS11)
8
Resonance Structures
(Forms, Hybrids)
• For two structures to be resonance structures
(representing the same species):
– The number and position of all atoms must be
identical (same skeleton structure)
– The total number of valence electrons must be
identical (otherwise two different species are being
represented!)
– At least one electron (or pair) has moved from one
atom or bond to another atom or bond
• I.e., the only thing that differs is the relative placement of the
valence electrons
Ppt23(PS11)
9
Equivalent Resonance Structures
• Example: O3 (next slide)
• Example: CO32- has three equivalent
resonance structures (draw them!):
NOTE: The structures are
analogous to those for NO3(shown on p. 380 of Tro),
because nitrate ion and
carbonate ion are isoelectronic!
Just remember that carbonate has a -2
charge, not a -1 charge.
Ppt23(PS11)
10
Significance / Interpretation of
Resonance Structures?
• Use to “patch up” an inadequate model!
• No single resonance structure is
consistent with observations!
– Experimentally determined structure for ozone
has two equivalent O-O “bonds”, each with a
length intermediate between the average O-O
bond and O=O bond!
Ppt23(PS11)
11
Figure 9.11 (b). Resonance
Interpretation
Ppt23(PS11)
12
Non-equivalent Resonance Structures
• Which is better?
→ To answer, assign “formal charges” (FCs) to each atom
→ Then assess which structure has more “zeros”, fewer
“charges”
O
N
C
N
O
O
O
N
N
Ppt23(PS11)
O
C
N
O
N
O
13
Formal Charges—Concept
• Each atom in an LDS can be assigned a formal
charge (FC)
– Analogous to assigning each atom in a “formula unit”
an “oxidation number”, but the manner (and purpose)
is different
• Did an atom “formally” gain or lose any electrons
as a result of the bonding arrangement
represented by the LDS?
– If so, that is considered “nonideal” to some extent
(takes energy)
• NOTE: Atoms (in LDS’s) get formal charges; The
LDS (as a whole) does not have a FC. The actual
charge on an ion is not a FC!
Ppt23(PS11)
14
How To Assign a FC to an Atom
• Compare the number of electrons that an atom
“wants” (i.e., to be neutral) to how many it
“has” in the LDS**.
–
–
–
–
0
If it has the same number, its FC = ___
-1
If it has one more electron, its FC = ___
-2
If it has two more, its FC = ___
+1
If it has one electron fewer, its FC = ___
**How do you COUNT the electrons here?
1) Both electrons in a lone pair clearly belong to the
atom they’re “on”.
2) ONE electron in a bond(ed pair) is given to each
atom involved in the bond. (“Cut the bond in half.”)
Ppt23(PS11)
15
Revisit Earlier Slide—Examples:
Assign FC’s to each atom in each LDS
FC’s: +1
WORSE
0
-1
C
O
O
FC
0
O
>
BEST
#v e-, if
neutral
#v e-,
in LDS
FC’s:
BETTER
0
C
2nd BEST (slightly) >>
0
O
WORST
0
+1
-1
-1
+1
0
-2
+1
+1
N
N
O
N
N
O
N
N
O
5
5
6
5
5
6
5
5
6
5
4
7
6
4
6
7
4
5
0
+1
-1
-1
+1
0
-2
+1
+1
Ppt23(PS11)
16
Which Resonance Structure is “Better”
(Lower in Energy)?
• If ALL atoms have ZERO formal charge and
octets, that is best! If not, assuming all atoms
have octets, the:
• Ones with fewer non-zero formal charges are
better
• Ones with smaller-magnitude non-zero formal
charges is better
• If all above are same (tied), then the one with a
negative FC on an atom farther to the right and
up on the periodic table (the atom with a greater
electronegativity--later) is better.
Ppt23(PS11)
17
Formal Charges tend to be “bad”
for “bad” skeleton structures!
• Try N2O with O in middle! (board)
• Try HCN with N in middle! (board)
• This provides a rationalization for the “lower
and to the left” goes in the middle” rule
about skeleton structures!
– Some instructors never even give that rule!
They make you figure out which is best by
doing all possible ones and picking the best
one based on formal charges!!
Ppt23(PS11)
18
“Formal Charge” idea rationalizes the
preferred LDS for BF3 (exception)
• See board
– My procedure would yield a B-F double bond
(B=F) to get “eight electrons around the
center”.
– But…this makes the FC on F a +1. “Bad”!
• Very hard to pull an electron away from F!
– High effective nuclear charge, right?
• (revisit this once electronegativity is defined)
Ppt23(PS11)
19
Formal Charges vs. Octet?
• Sometimes the Resonance Structure with the
Best Formal Charges has atoms with more than
8 electrons around it.
– i.e., sometimes, to minimize FCs, you need to “break”
the octet rule (see examples, below)
• In such cases, I would never ask you “Which is
best?” without clarifying
– With respect to octet rule?
– With respect to formal charges?
• Examples: SO2, SO42- (on board)
Ppt23(PS11)
20
Special Case Scenarios
(simple organic compounds)
• See next slide →
Ppt23(PS11)
21
Molecules with C, N, O, and “H”
(hydrogen or halogen)
• C, N, and O are in the 2nd row: don’t go over 8 e-’s
• In organic compounds (C & H present), these
atoms usually follow the “simple” patterns below
[NOTE: these patterns will lead to both “octets”
(duet for H) and formal charges of ZERO for all atoms!]
•
•
•
•
•
C has 4 v e-’s: forms 4 bonds and has no lone pairs
N has 5 v e-’s: forms 3 bonds and has one lone pair
O has 6 v e-’s: forms 2 bonds and has 2 lone pairs
Halogen, 7 v e-’s: forms 1 bond, has 3 lone pairs
H has 1 ve:
forms 1 bond, no lone pairs
Ppt23(PS11)
22
Example
Create an LDS from the skeleton structure below by
adding lone pairs AND POSSIBLY MAKING DOUBLE OR
TRIPLE BONDS so that the octet rule AND the simple
patterns for C, N, and O are “satisfied”:
• Because O “wants”
Cl
H
H
O
H
two bonds (and two
lone pairs)
N
C
C
C
N
C
C
C
H
C on left:
N
• Has 3 bonds; “wants”
H
4 bonds, no lone pairs
• Make bond “down” because otherwise, H
N “up” would have 4 bonds when N
“wants” 3 bonds and one lone pair
Ppt23(PS11)
O
H
H
• Add lone pair to N,
move to next C
23