11/14/2014
A polar covalent bond results from unequal sharing of
electrons, and leads to partial charge separation. Which
bond places the largest δ+ charge on a Ge (Z = 32) atom?
a) Ge‐H
d) Ge‐Br
b) Ge‐F
e) Ge‐I
The electrostatic potential maps below correspond to
the molecules HNO, FNO, and NO2. Which one is HNO?
a)
c) Ge‐Cl
The magnitude of the δ+ and δ– charges, and the %
ionic character in a polar covalent bond, increases as
the electronegativity difference (ΔEN or Δχ) between
the atoms increases. General trend: χ ↑ up and to
the right (but χH ≈ χC), so Ge‐F has the largest Δχ.
1
χGe = 2.0
χH = 2.2 χI = 2.7 χBr = 3.0 χCl = 3.2 χF = 4.0
2
b)
blue = highly δ+ red = highly δ–
blue = lower χ red = higher χ
χ values: H 2.2, N 3.0, O 3.5, F 4.0
HNO: H has lowest χ, it should appear as intense
blue region on one end, compared to red N/O.
NO2 and FNO: O and F have higher χ than N, so the
N will be slightly blue in the middle.
Which pair of structures are resonance forms?
Which is the correct Lewis structure for the
nitrate ion, [NO3]– ?
b)
O
H
a)
a)
c)
b)
c)
H
O
H
O
P
O
O
c)
N
d)
O
O
O
H
P
O
H
O
H
d)
e)
O
3
• [NO3]– has 5 + 3x6 + 1 = 24 valence e–
• N has lowest χ so it is central, surrounded by three O
• 24e completes three BP + nine LP on Os
• N requires octet, so one double bond is formed by
shifting one O LP to an O‐N BP
e) there is more than one correct response
4
a) changes atomic connectivity: isomers, not resonance
b) same structures, rotated 180°
c) rearrangement of electrons: resonance forms
d) same structures, flipped
1
11/14/2014
What is the formal charge on C atom in the
Lewis structure of CO?
Which is the best Lewis structure for
nitryl fluoride, FNO2 ?
a) –2
b) –1
c) 0
d) +1
e) +2
5
a)
F
b)
N
F
N
O
O
O
c)
F
O
d)
N
F
O
CO is highly unusual. It is one of the few stable covalent
molecules in which a C atom forms fewer than four
bonds. With 5 neighbouring electrons, it carries a –1
formal charge, which is also unusual for stable carbon
compounds.
6
Bond
Order
Bond Length
/pm
Bond Energy
/kJmol–1
1
145
142
b) O3
1.5
128
303
c) O2
2
121
495
Lewis structure
a) H2O2
N
O
a) no octet at N, large FCs (boo!)
b) satisfies all octets, (–) FC on higher χ O,
(+) FC on lower χ N
c) (+) on highest χ F! (boo!)
d) exceeds octet at N! (boo!)
None of the following compounds has lone pairs on the
central atom. Which has the largest bond angle?
Which has the shortest O‐O bond length?
a) BCl3
b) CCl4
c) PCl5
d) SF6
d) they should all be about equal length
7
O
O
+2
8
No LPs means all AXn structures, with shape
determined by expected arrangement of atoms.
AX2 – linear: 180°
AX3 – trigonal planar: 120°
AX4 – tetrahedral: 109.5°
AX5 – trigonal bipyramidal: 120° and 90°
AX6 – octahedral: 90°
2
11/14/2014
What electron pair arrangement and molecular
geometry are predicted for the triiodide anion, [I3]–?
a)
b)
c)
d)
e)
What is the most reasonable predicted bond angle for
ozone, O3 ?
linear arrangement, linear geometry
trigonal planar arrangement, bent geometry
tetrahedral arrangement, bent geometry
trigonal bipyramidal arrangement, linear geometry
none of the above
a)
b)
c)
d)
e)
3x7 + 1 = 22e
AX2E3 is 5 electron pair regions:
trigonal bipyramidal arrangement
3 LPs in the equatorial positions, leaving atoms axial:
linear geometry of atoms
9
AX2E: a perfect AX3 trigonal planar arrangement would
be 120°, but the LP takes up more space and causes
greater repulsion than the BP electrons, so the bond
angle contracts slightly.
10
Which ion has the smallest O‐N‐O bond angle?
O
a) [NO2]+
b) [NO2]–
c) [NO3]–
d) [NO4]3–
11
106°
109°
117°
120°
180°
N
angle BO length
O
N
O
O
O
180°
2
115pm
a)
b)
c)
d)
115° 1½ 124pm
N
O
Which compound has the largest molecular dipole?
χ values: H 2.20 C 2.55 I 2.66 Br 2.96 Cl 3.16
O
O
O
N
O
O
120° 1⅓ 122pm
109.5° 1
139pm
Bond angle prediction requires VSEPR, and VSEPR
requires a Lewis structure. Draw the Lewis structures!
[NO2]+ is 16e, AX2: predicts 180°
[NO2]– is 18e, AX2E: predicts <120°
[NO3]– is 24e, AX3: predicts 120°
[NO4]3– is 32e, AX4: predicts 109.5°
12
μ=0D
CH4
CH3Cl μ = 1.90 D
CH3Br μ = 1.82 D
CH3I μ = 1.64 D
–
X
+
H
+
C
H
H
+
All these compounds have an AX4 tetrahedral
geometry. The C‐H bonds are very slightly polarized
with δ+ on H. As the C‐X (X = H I Br Cl) bond polarity
increases, the molecular dipole also increases. The C‐Cl
bond has the largest Δχ, so CH3Cl has the largest dipole
moment.
3
11/14/2014
Which compound has the smallest molecular dipole?
χ values: Ga 1.81 As 2.18 P 2.19 N 3.04 Cl 3.16
a)
b)
c)
d)
NCl3 μ = 0.6 D
PCl3 μ = 1.0 D
AsCl3 μ = 2.0 D
GaCl3 μ = 0 D
GaCl3 is AX3, trigonal planar with 120° angles.
Individual Ga‐Cl bonds are polar, but the bond dipoles
cancel, leaving the molecular non‐polar.
13
The other EX3 are pyramidal, and polar.
4