Lecture 25: VSEPR
• Reading: Zumdahl 13.13
• Outline
– Concept behind VSEPR
– Valence Shell Electron-Pair Repulsion (VS EPR)
– Molecular geometries
• Both LDS and VSEPR have (backing them up) Quantum
Mechanics, which tells us that electrons are paired in orbitals
and bonding electrons are in molecular orbitals between atoms.
• Problems (Chapter 13, Zumdahl 5th Ed.)
• 59 (and predict shapes), 71, 72, 73, 74, 83, 85, 86, 88,
89, 90
1
LDS and VSEPR
• The Lewis Dot Structure (LDS) approach provided
some insight into molecular structure in terms of
bonding, but what about geometry (shape of
molecules)?
• Recall: there are two types of LDS electron pairs:
bonding (BP) and lone (LP).
• Valence Shell Electron Pair Repulsion (VSEPR)
states: The 3D structure of a molecule can be
determined by minimizing the interaction between
electron pairs.
2
Molecular Shapes
• Example: CH4
H
H C H
H
Lewis Structure
3D Structure: tetrahedral
VSEPR says it should be tetrahedreal: Why? Because the tetrahedral
arrangement of the 4 bonding pairs of electrons minimizes the electron
electron interactions; provides the maximum distance between electron
pairs.
3
Steric Number: A concept for molecular geometry
• The steric number is the number of atoms plus lone
pairs around a central atom
•Example: NH3 (both bonding and lone pairs).
H
H N
H
Lewis Structure
VSEPR Structure
Steric number is 4 (like CH4); just one H is missing. The
electrons are still arranged nearly tetrahedrally about the N; but
the molecule, defined by where the atoms are, is not tetrahedral.
The molecule has a trigonal-pyramidal shape.
4
Application of VSEPR
Each electron pair is in its own orbital, whether or not
it bonds to another atom. The orbital belongs to
the molecule, so this orbital is a Molecular Orbital
(M.O.)
The previous examples illustrate the strategy for
applying VSEPR to predict molecular structure:
1. Construct the Lewis Dot Structure
2. Arranging bonding/lone electron pairs
in space such that repulsions are minimized.
5
Minimize Electron-Pair Repulsions
• Linear Structures: angle between bonds is 180°
• Steric number is 2
Orbital;
• Example: BeF2
bonding pair.
F Be F
F
Be
F
F Be F
180°
Orbital showing where the
bonding pair of electrons
resides.
6
Three pairs around B (steric # = 3)
• Trigonal Planar Structures: angle between
bonds is 120°; arrange 3 things in a plane equally
spaced from a center point. Get trigonal planar.
F
• Example: BF3
F
F
B
120°
FB
F
F
F
B
F
F
7
Arrangement for Symmetry:
Steric Number is 4
• Tetrahedral: Any (and every) HCH angle
between bonds is: θT 109.5° ⇔ cos θT = − 13
• Example: CH4
H
109.5°
H C H
H
Arrange 4 things in 3-D space to be equidistant from a
central point. They are tetrahedrally arranged. Here the
molecular structure and the bonding pairs of electrons have
the same shape (because no lone pairs on central atom).
8
LPs repel more than EPs
• Pyramidal: Bond angles are less than 109° (and
certainly less than 120°), and structure
is nonplanar:
• Example: NH3
H
H N
H
107°
9
VSEPR Background: Compare LP and BP
• Tetrahedral: angle may vary from 109.5°
(exactly) due to size differences between bonding
and lone pair electron densities (implies orbitals).
bonding pair
lone pair (Takes up more
space than a bonding pair.)
10
Steric Number 4 (two LP and two BP)
• Classic example of tetrahedral angle shift:
from perfect tetrahedral (109.5°) to real water:
11
Steric Number (SN): 4 CH4, NH3, and H2O
Tetrahedral
(trigonal) pyramidal
bent
The molecules are not tetrahedral (only CH4 is);
However, in all cases the electrons (octet) are ~tetrahedrally
arranged about the central atom. Progressive LP repulsion.
12
Steric Number 5 shapes (PCl5)
• Trigonal Bipyramidal, three orbitals are120°
apart in plane, and two orbitals are 90° to plane:
XY Plane
• Example, PCl5:
Cl
90°
Cl
Cl
P
Cl
120°
Cl
Compare with SF4 and ClF3
13
VSEPR and Octahedral Shape
• Octahedral: all nearest neighbor angles are 90°
•4 Cls are in the x-y plane with the S and the
other two are up and down on the z axis.
•But all Cls are equivalent
• Steric Number is 6.
• Example, SCl6:
90°
14
Advanced VSEPR (Steric Number 6)
• XeF4: Steric Number 6 (Same as SCl6)
• Geometry: Square Planar versus “See Saw”
See Saw
Square Planar
No dipole moment
15
Advanced VSEPR (SN=5)
• Driving force for last structure was to
maximize the angular separation of the lone pairs.
Lone Pairs need to have more room and be further
from other pairs than bonding pairs.
16
Advanced VSEPR (SN=3)
• VSEPR and resonance structures. Must look at
VSEPR structures for all resonance species to
predict molecular properties.
O
O
O
O
O
O
O
17
VSEPR
• Provide the Lewis dot and VESPR
structures for CF2Cl2. Does it have a dipole
moment?
F
F
32 e-
Cl
C
Cl
F
F
Cl
Cl
Tetrahedral
18
Practice (Z13.47,48)
•
•
•
•
Take a molecule: H2CO (formaldehyde)
:O :
&
Draw the LDS
H − C −H
Find the Steric Number ( Here 3)
From SN choose geometry (eg Planar,
Triangle)
• Determine Molecular Geometry; planar,
n = 120°
bond angle: HCO
• Dipole: Yes, along the C=O axis and
negative at the O end (like water)
19
Practice (Z13.73,74)
• Which molecules in problems 59 and 60 are
linear, (with 3 LP), T shapes or See-Saw.
• Rather do each molecule, recognize that the
Steric Number must be 5.
– For linear you have two atoms and 3 LP,
– For T shape, have 3 atoms and two LP
– For See-Saw have 4 atoms and 1 LP.
PF5
SN 5
LP 0
BeH 2
Br3−
SF4
2
0
5
3
5
1
XeF4 ClF5
6
2
6
1
SF6 ClF3
6
0
5
2
From the table, from Z13.59, Br3- is linear, ClF3 is T, and SF4 is seesaw. BeH2 is linear but no LP, and PF5 is trigonal-bipyramidal. 20
Practice: Z13:47, 48, 50, 59, 71, 72, 73
Practice making LDS, arranging electrons, finding Steric
#, and deciding on bond angles and molecular geometry.
Same molecules but grouped to see VSEPR trends:
=
4
SO
SF4
PF3
+
2
NO
−
2
IF
( cf . SF6 )
( cf . PF5 )
2
NO2 NO
−
4
IF
IF5
O3
IF7
21