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VSEPR Theory Chem 221 Tuesday, July 12, 2011 Molecular Geometry • Hybridization– mixing of atomic orbitals (s, p, d...) to form hybrid (sp, sp2...) orbitals of different shapes and energies. The ThreeDimensional Symmetry of sp3 Hybrid Orbitals Hybridization of one s and three p orbitals gives four sp3 orbitals. sp3 orbitals have equivalent shapes and energies, but different orientations; each points to a vertex of an imaginary tetrahedron. • Bond angles are 109.5°. •
• 2px orbital
z
y 2s orbital x+
2py orbital 2pz orbital
mixing of the
2s, 2px ,2py and 2pz orbitals yields four sp3 hybrid
orbitals directed
towards the corners
of a tetrahedron Hybrid Orbitals and Molecular Shape
CH4 sp3 hybridization The ground state electron configuration of C is 1s2 2s2 2p2 or 1s valence level 2s 2px 2py 2pz The Shape of CH4
H
1s H Inphase combination
of atomic orbitals
yields bonding
molecular orbitals. sp3
sp3 C
sp3 H 1s sp3 1s H 1s
H H o 109.5
H
H H C
H four sigma bonds H C
H a tetrahedral geometry sp2 hybrid orbitals project in a plane
• Hybridization of one s and two p orbitals give three sp2 orbitals. z z
y 2px orbital
x + xy plane 2s orbital
2py orbital
mixing of the 2s, 2px and 2py orbitals sp2 hybridized carbon yields three hybrid
sp2 orbitals Spatial arrangement of sp2 sp2 orbitals have equivalent shapes and energies, but different orientations in space. Each points to a different vertex of an imaginary triangle. • sp2 hybridization leaves one p orbital unchanged. • sp2 orbitals are trigonal planar; the leftover p orbital is perpendicular to the
sp2’s. • pz 2px 2py 2pz energy 2s sp2 2pz hybridization sp 2 sp 2 sp 2 valencelevel orbitals 1s
ground state
of C 1s sp2
C
sp2
bonding orbitals Three equivalent sp 2 orbitals
are in the XY plane. The idealized
interorbital angle is 120o . sp Hybrid Orbitals have Directionality
• Hybridization of one s and one p orbitals gives two sp orbitals. z
y x 2s orbital + 2px orbital mixing of the 2s and 2px yields
two 2spx orbitals Hybrid Orbitals and Molecular Shape BeCl2 sp hybridization
The ground state electron configuration of Be is 1s2 2s2 or 2s 1s valence level
Cl Be Cl two sigma bonds
2py 2pz Cl
Cl Be
a linear structure 2spx 2spx •
1s Two p orbitals remain unchanged. Hybridization • sp orbital • 50% s character, 50% p character • sp2 orbital • 33% s character, 66% p character • sp3 orbital • 25% s character, 75% p character Summary of Important concepts 1) 2)
3)
4) 5)
6) An atomic orbital (AO) corresponds to a region of space about the nucleus of a single atom where there is a high probability of Sinding an electron. s orbitals are spherical, p orbitals are like two almost
tangent spheres. Orbitals can hold a maximum of two electrons when their spins are paired When atomic orbitals overlap, they combine to form molecular orbitals
(MOs) The number of molecular orbitals always equals the number of atomic
orbitals from which they are formed Hybrid atomic orbitals are obtained by mixing (hybridizing) the wave
functions for orbitals of different types (i.e., s and p orbitals) but from the
same atom A sigma (σ) bond (a type of single bond) is one in which the electron
density has circular symmetry when viewed along the bond axis A pi (π) bond, part of double and triple carbon–carbon bonds, is one in
which the electron densities of two adjacent parallel p orbitals overlap
sideways to form a bonding pi molecular orbital Valence Shell Electron Pair Repulsion Theory
VSEPR theory provides a simple way to predict molecular
geometries around a central atom. These predictions are
consistent with hybrid molecular orbital theory. Key Features of VSEPR Theory
(1) Molecules or ions may be analyzed where a central atom is
covalently bonded to two or more atoms or groups. (2) All pairs of valence electrons around the central atom are counted:
bonding and nonbonding. (3) Because of electronelectron repulsion, pairs of electrons tend to
stay as far apart as possible. Repulsion due to nonbonding pairs
of electrons is greater than repulsion due to bonding pairs.
(4) The preferred geometry has all pairs of valence electrons as far
apart as possible to minimize repulsion among electron pairs. Examples Methane :
:
: : H
HCH
H
There are four pairs of bonding electrons
in the valence level around the central
carbon. Maximum separation of the four
pairs is achieved with a tetrahedral
geometry where each electron pair points to
the corner of a tetrahedron. H
H C 109.5o
H
H an idealized
tetrahedral
geometry The Importance of Nonbonding Electron Pairs : Ammonia H NH
H The geometry of ammonia is
often described as a trigonal
pyramid. This shape is predicted
by VSEPR theory. There are four pairs of electrons in the valence level of the
central atom: three bonding pairs and one nonbonding pair. VSEPR theory predicts a tetrahedral geometry around nitrogen. : HN
H 107o H The actual geometry is a
distorted tetrahedral with a
slight compression of each
HN
H HNH angle in order to
H
provide more space for the
trigonal pyramid
nonbonding electron pair. Water: Two Nonbonding Electron Pairs The water molecule is bent as
predicted by VSEPR theory. H2O The are four pairs of electrons in the valence level around the
central oxygen atom: two bonding and two nonbonding. :
HO :
H H O
105o
bent H VSEPR theory predicts the four pairs
will be projected to the corners of a
tetrahedron for maximum separation. The actual geometry is a distorted
tetrahedron with the HOH bond angle
compressed to 105o to provide more space
for the nonbonding electron pairs. Boron Trifluoride: Three Bonding Pairs
BF3 F BF
F Boron trifluoride has a trigonalplanar
geometry as predicted by VSEPR theory. There are three pairs of bonding electrons in
the valence level around the central boron. VSEPR theory predicts the three pairs of electrons will be
projected to the corners of an equilateral triangle for maximum
separation of the electron pairs. F
B
F F
120o trigonal planar Berylium Chloride: Two Bonding Pairs BeCl2 Cl Be Cl Berylium chloride has a linear geometry as
predicted by VSEPR theory. There are two pairs of bonding electrons in the
valence level around the berylium atom. VSEPR theory predicts the two pairs will be projected in opposite
directions along an axis for maximum separation of the electron pairs. 180o Cl Be Cl
linear Multiple Bonds: Sets of Electrons
VSEPR theory correctly predicts the geometry around central
atoms that have four and six electron bonds. The multiple
bond is treated as a single set of electronsa single unit. The
preferred geometry provides maximum separation of all sets
and pairs of electrons. Carbon Dioxide CO2 Two Sets Carbon dioxide has 16 valence electrons. To satisfy the octet rule ,
two carbonoxygen double (four electron) bonds are required. 180o C
linear :: :: O O VSEPR theory treats each four electron
bond as a set. Maximum separation of
the two sets around the central carbon
is achieved in a linear geometry. Ethyne (Acetylene) C2H2 One Set and a Pair Ethyne has 10 electrons in the valence level. To satisfy the octet
rule, a six electron (triple) bond is required between the carbon
atoms.
180o 180o HC CH VSEPR theory predicts a linear geometry
for maximum separation of the electron set
and single pair of electrons around each
carbon. Quiz 1.019
Use VSEPR theory to predict the geometry around the carbon atom
in each of the following chemical species. :: H2C=O formaldehyde H3C+
methyl cation  H3C: methyl anion ...
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This note was uploaded on 08/06/2011 for the course CHEM 221x taught by Professor Broyer during the Summer '11 term at USC.
 Summer '11
 BROYER
 Atom, Mole, VSEPR

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