Transcript VSEPR PowerPoint - Tetrahedral, Trigonal Bipyramidal, etc.

Valence
Shell
Electron
Pair
Repulsion
Theory
Planar
triangular
Tetrahedral
Trigonal
pyramidal
Bent
Molecular Shape
VSEPR theory assumes that the shape of a
molecule is determined by the repulsion of
electron pairs.
VSEPR Theory
• Based on Lewis structures
• Theory predicts shapes of compounds
• VSEPR (pronounced “vesper”) stands for
Valence Shell Electron Pair Repulsion
• VSEPR predicts shapes based on electron pairs
repelling (in bonds or by themselves)
• Electrons around central nucleus
repel each other. So, structures
have atoms maximally spread out
VSEPR overview
• Each shape has a name
• Names of Shapes:
• tetrahedral
• trigonal pyramidal
• Bent
• Linear
• trigonal planar
Models
Tetrahedral
Triangular Planar
Trigonal pyramidal
Bent or V
Linear
H
H C H
methane, CH4
H
Tetrahedral
109.5°
Bonds are all evenly spaced electrons
H
..
H C H
..
H N H
..
..
H
H
ammonia
NH3
Trigonal
Pyramidal
Less repulsion between the bonding pairs of electrons
H
H C H
..
H N H
H
H
109.5° (109.5°) 109.5° (107°)
..
H O
.. H
water, H2O
109.5° (104.5°)
H
C H
H
H
N H
H
H
H
O
H
Bent or V
2 unshared pairs of e’s
at top of O repel bonds
and force them to bend
Molecule
Lewis Structure
Number of
electron pairs
SHAPE
Tetrahedral
CH4
NH3
4
4
Trigonal
Pyramidal
(3 shared
1 lone pair)
Molecule
Lewis Structure
Number of
electron pairs
SHAPE
Bent
H2O
4
(2 shared
2 lone pairs)
Linear
CO2
2
Molecule
Lewis Structure
Number of
electron pairs
SHAPE
Linear
BeCl2
BF3
2
3
Trigonal
Planar
Hybrid Orbitals
• VSEPR works well for shapes/geometry,
but not for describing the types of bonds
formed
• Hybridization: the mixture of atomic
orbitals to form the same number of new
orbitals
• Carbon is the most common element that
undergoes hybridization
Methane Hybrid Orbitals
• Electron configuration of methane: [He]2s22p2
• One s and three p orbitals hybridize to form
four sp3 orbitals
Molecular Shape/Hybrid Orbitals:
Linear/sp
Trigonal planar/sp2
Tetrahedral/sp3
Trigonal pyramidal/sp3
Bent/sp3
Phosphorus trihydride
• Total number of valence electrons: 8
• Lewis structure: three single bonds and one
lone pair
• Shape: trigonal pyramidal
• Four bonding positions = sp3 hybrid
Intermolecular Forces
• Forces that hold together identical particles
such as water molecules in a drop of water
• Three such forces are: dispersion forces,
dipole-dipole forces, and hydrogen bonds
Dispersion Forces
• Also known as London forces
• Weak forces that result from a temporary shift
in the density of electrons in electron clouds
• For example, if two nonpolar molecules collide,
the electron clouds of one molecule repels the
electron cloud of the other molecule, creating a
greater electron density in on region of each
electron cloud
Dipole-Dipole Forces
• Attractions between oppositely charged regions
of polar molecules
• Since dipoles in polar molecules are permanent,
dipole-dipole forces are stronger than dispersion
as long as the molecules are similar in mass.
Hydrogen Bonds
• Type of dipole-dipole attraction
• Occurs between molecules containing a
hydrogen atom bonded to a small, highly
electronegative atom with at least one lone pair
(i.e., fluorine, oxygen, or nitrogen)
• F, O, and N are electronegative enough to cause
a large partial positive charge on the H, but
small enough to allow their lone pairs to come
close to H atoms