vsepr powerpoint - tetrahedral, trigonal bipyramidal, etc
TRANSCRIPT
LEARNING OBJECTIVES
• Apply VSEPR to predict electronic
geometry and shapes of simple
molecules
• Distinguish between polar and nonpolar
bonds in molecules
• Predict polarity of simple molecules
from bond polarity and molecular shape
TOTAL NUMBER OF GROUPS
DICTATES ELECTRONIC GEOMETRY
Octet rule:
Two – linear
Three – trigonal planar
Four – tetrahedral
Additional possibilities (expand octet):
Five – trigonal bipyramidal
Six - octahedral
Electronic geometry considers
bonded atoms only.
Molecular geometry considers
unbonded pairs as well
MOLECULAR GEOMETRY
Molecular geometry is the three-dimensional
arrangement of a molecule’s atoms in space.
Linear
Bent
Trigonal-planar
Tetrahedral Trigonal-
pyramidal
Trigonal-bipyramidal Octahedral
VALENCE
SHELL
ELECTRON
PAIR
REPULSION
THEORY
VSEPR theory assumes that the shape of a
molecule is determined by the repulsion of
electron pairs.
VSEPR theory states
that repulsion between
the sets of valence-level
electrons surrounding an
atom causes these sets
to be oriented as far apart as possible.
Molecular Shape
VSEPR THEORY
• VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion
• Based on Electron Dot (Lewis structures) • Theory predicts shapes of compounds based on electron pairs repelling (in bonds or by themselves)
• Electrons around central nucleus repel each other. So, structures have atoms maximally spread out
• Each shape has a name (you will have to memorize these)
• tetrahedral • trigonal pyramidal • bent • linear • trigonal planar
VSEPR AND MOLECULAR GEOMETRY
VSEPR AND MOLECULAR GEOMETRY
Molecule Lewis Structure Number of
electron pairs
CH4
NH3
SHAPE
Tetrahedral
Trigonal
Pyramidal
4
4
(3 shared
1 lone pair)
Molecule Lewis Structure Number of
electron pairs
H2O
CO2
SHAPE
Bent or V
4
(2 shared
2 lone pairs)
2
Linear
Molecule Lewis Structure Number of
electron pairs
BeCl2
BF3
SHAPE
2
3
Linear
Trigonal
Planar
Use VSEPR theory to predict the molecular
geometry of boron trichloride, BCl3.
First write the Lewis structure.
Boron is in Group 13 and has 3 valence
electrons.
Chlorine is in Group 17 so each chlorine atom
has 7 valence electrons.
The three B-Cl bonds stay farthest apart by pointing to the
corners of an equilateral triangle, giving 120o angles between
the bonds.
This would be trigonal-planar geometry.
methane, CH4
Bonds are all evenly spaced apart
Tetrahedral
109.5○
Less repulsion between the bonding pairs of electrons
.. ammonia
NH3
Trigonal Pyramidal
●●
surprise: the lone pairs occupy more space than the bonded atoms (with very few exceptions)
Two unbonded pairs of electrons make bond angles slightly less than tetrahedral due to
greater repulsion
.. water
H2O
bent
O H H
Bonded electrons can take maximum position apart – 180°
.. Carbon dioxide
CO2
linear
No lone pairs of electrons allows maximum bond angle in 1-D plane
.. Barium floride
BF3
Trigonal planar
The “z” plane – 3-D
22
The geometry of a
molecule refers to
the positions of
atoms only.
Water, H2O, has two unshared pairs, and its
molecular geometry takes the shape of a
“bent” or angular molecule.
Bent
• Unshared electron pairs repel other electron
pairs more strongly than bonding pairs do. • This is why the bond angles in ammonia and water are
somewhat less than the 109.5o bond angles of a perfectly
tetrahedral molecule.
The strongest intermolecular forces exist
between polar molecules.
•Polar molecules act as tiny dipoles. A dipole is created by equal but opposite charges that
are separated by a short distance. •The direction of a dipole is from the dipole’s positive pole to
its negative pole.
•A dipole is represented by an arrow with its head pointing
toward the negative pole and a crossed tail at the positive
pole.
•The dipole created by a hydrogen chloride molecule is
represented below:
MOLECULAR POLARITY AND DIPOLE-DIPOLE
FORCES
H Cl
The negative region in one
polar molecule attracts the
positive region in adjacent
molecules. So the molecules
all attract each other from
opposite sides.
The forces of attraction
between polar molecules
are known as dipole-
dipole forces.
Which atom attracts e- more?
H ― Cl δ+ δ-
electronegativities 2.1 3.0
C = O H H
2.5 3.5 2.1
2.1
O = C = O