Molecular Geometry - chemed.tamu.educhemed.tamu.edu/chem101/pptnotes/geometry HO 6pp 13p.pdfValence Bond (VB) Theory (orbital overlap model) ... Dr. Williamson’s Molecular Geometry

Dr. Williamson’s Molecular Geometry Notes

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Molecular Geometry

Dr. V.M. WilliamsonTexas A & M UniversityStudent Version

Valence Shell Electron Pair Repulsion-VSEPR

1. Valence e- ___________to some extent 2. Electron pairs move as far away as possible

to ________________ repulsive forces3. Shape determined by repulsive position of

electron pairs to give base or electron geo. MUST consider both lone pairs and bonds.

4. Molecular geo from base or electron geo!  If no lone pairs (only bonds), then electron geo _____

molecular geo5. Molecular geometry describes the position

of the bonded atoms around the central atom.

2 Principal originator: R. J. Gillespie in the 1950’s

VSEPR: Definition of Terms"  Central atom: bonded to more than

one other atom"  Regions of high electron density:

bonding electron pair = ___ region (multiple bonds count = ___ region)lone pair of electrons = ___ region

"  Electronic geometry: geometry of regions of high electron density around central atom

"  Molecular geometry: geometry of atoms around central atom

Determining Geometry1. Count the regions of electron density

from Lewis structure. (lone pairs and single, double or triple bonds are just 1 region.)

2. Determine the "base" or electron geometry.

3. Determine the number of electron regions that are bonded to another atom.

4. Determine the actual molecular geometry from the "base" or electron geometry

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"  To minimize repulsions, situate electron pairs as far apart as possible

VSEPR: Electronic Geometries

109.5°

109.5°

O

VSEPR

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2

7 4 regions 5 regions 6 regions

Electronic Geometries in 3-D

Copyright © 1995 by Saunders College Publishing

"  Lone pairs occupy ______ space than bonding pairs

VSEPR: Space Considerations

>109.5°

<109.5°

O H H

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"  Multiple bonds treated as single bonds but they occupy ______ space

VSEPR: Multiple Bonds

Predict 120° actually 117°

Multiple Bonds

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3

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Electronic vs Molecular Geometry"  When nonbonding or lone electron pairs are

present, electronic and molecular geometries are not identical


CH4SF4

SF4 CH4

Electronic and Molecular Geometries 4 regions of Electron Density

15 16

5 regions of electron density

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5 Regions of Electron Density

6 Regions of Electron Density

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4

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VSEPR: When There is More Than One Central Atom"  Determine molecular geometry about

each central atom to obtain complete structure of the molecule

C C H

H H

H H

H

Tetrahedral Carbons Copyright © 1995 by Saunders College Publishing

VSEPR: Another Example109° 120°

lone pair bonding electrons

caffeine

VSEPR: A Look at Caffeine


Predict the shape and the approximate bond angles for the atom numbered 1 and 2.

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(A) linear, _____ and tetrahedral, ____ (B) ____________, 109.5 and ____________, 109.5 (C) angular, ____ and square planar, ____ (D) __________, 109.5 and ___________, 109.5

Bond Polarity and Molecular Polarity

"  Bond polarity can be determined by examining the difference in EN for the 2 atoms in the bond

"  Molecular Polarity is the sum of the bonds in the molecule. This sum depends on the ________________________!!

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5

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Measurement of Molecular Polarity

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Molecular Polarity from Bond Polarity and Geo

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Molecular Polarity:Symmetrical Molecular Geometries

µ total = 0

Symmetry and Polarity

30 CH4 CClH3


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Dipole Moment"  A measure of the polarity of a molecule"  Defined as the product of the magnitude of the

partial charges δ + and δ – and the distance d separating these charges

"  Represented by symbol µ in unit of debye (D)

H Cl δ+ δ-

dipoles

µ = 1.07 D ∴ polar molecule

dipole moment

Dipole Moment: An Application"  The dipole moment of a molecule can be

used to provide, indirectly, information on molecular geometry

Y X Y Linear

Y X Y

Net dipole = 0 ∴ Nonpolar

Y X Y Bent

Y X Y

Net dipole > 0 ∴ Polar

Polarity of Water


See demo

Why is water attracted to this comb? (Try it at home J)

35 36


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Why polarity??

"  Solubility!!"  See demo"  Likes dissolve _____"  Ionic and polar covalent substances

dissolve in polar solvents! Think of ionic as polar extreme!!

"  Nonpolar substances dissolve in nonpolar solvents

"  What will dissolve in water? In CCl4?37

Valence Bond (VB) Theory(orbital overlap model)"  A bond is formed by the overlap of an

orbital from 1 atom with an orbital from another atom. This theory allows for mixing of orbitals (hybridization) to fit the shape and energies observed in the molecule.

Principal originator: L. Pauling in the 1930’s & 40’s

Hybrid Orbital Formation


• Explains why Carbon will form 4 bonds • Carbon = 2s2 2p2

• ↑↓ ↑ ↑

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41 42


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Hybrid Orbitals:Linear Geometry

Why does BeCl2 form?Be is 2s2:↑↓ 2s 2p

In order to make 2 bonds, need 2 places so:

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180 degrees

Hybrid Orbitals: Trigonal Planar Geometry

Why does BF3 form? B is 2s22p1 : ↑↓ ↑ 2s 2p

In order to make 3 bonds, need 3 places so:

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120 degrees

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90 & 120 degrees

P is 3s23p3 : ↑↓ ↑ ↑ ↑ 3s 3p

To make PF5

Note: ___ empty, unhybridized d orbitals

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90 degrees

To make SF6 Note: __ empty, unhybridized d orbitals

S is 3s23p4 : ↑↓ ↑↓ ↑ ↑ 3s 3p


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Electronic Geometries andHybridizations of Central Atom

High Electron Density Regions

Electronic Geometry

Hybridized Atomic Orbitals

Hybridi- zation

2 linear 1 s, 1 p sp

3 trigonal 1 s, 2 p’s sp2 planar

4 tetrahedral 1 s, 3 p’s sp3

5 trigonal 1 s, 3 p’s, 1 d sp3d bipyramidal

6 octahedral 1 s, 3 p’s, 2 d’s sp3d2 50

Predict the hybridization at the numbered atoms in the following molecule.

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(A) sp, ____ (B) ___, sp3 (C) ___ , sp2 (D) sp3, __

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Predict the hybridization for # ___: (A) ____ (B) ____ (C) ____ (D) _____

Sigma (σ) Bond FormationSigma (σ) Bond Formation


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Pi (π) Bond Formation

σ and π bonding

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OR

C2H4

Hybrid Orbitals:Compounds with Double Bonds

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11

Triple Bonding

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How many sigma and pi bonds are in:

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Molecule (b)?? (A) ____, 1 (B)  ___, 1 (C)  ____, ___ (D)  ____, 1

Molecule (c)?? (A)  ___, 2 (B)  ___, 3 (C)  ___, 2 (D)  ___, 3

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Benzene C6H6

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Benzene C6H6

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Benzene C6H6

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"  VSEPR = Repulsion between e- pairs such that distance was maximized and effect of repulsion minimized determines _______.

"  Valence Bond = atomic orbitals overlap to form bonds. Atomic __________ orbitals can form.

"  Both FAILED to explain:! why O2 was attracted to a magnet (was

paramagnetic) ! Or why SO2 had 2 equivalent bond that are an

intermediate between a double and single bond.

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Liquid N2 and O2 with a Magnet

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Molecular Orbital Theory"  MO theory = molecular orbital theory.

Assumes that new MOLECULAR orbitals are formed from the atomic orbitals of the bonding atoms. These molecular orbitals can reflect the geom. of the molecule, but are hard to visualize.

71 72

Bonding molecular orbitals (MO’s) = e- spend most of time between the two nuclei.

Antibonding MO’s = e- spend most of time on the outside of nuclei.

Consider H2


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MO Energy Level Diagrams H2 through N2 NEW filling order

Ener

gy Molecularorbitals

2p 2p

π2p

σ*2p

π*2p

1s 1s σ1s

σ*1s

2s σ2s

σ*2s

Atomic orbitals

2s

σ2p

MO Energy Level Diagrams O2 through Ne2 NEW filling order

Ener

gy Molecular orbitals

2p 2p

1s 1s σ1s

σ*1s

σ*2p

σ2p

π*2p

2s 2s σ2s

σ*2s

Atomic orbitals

π2p

How Is O2 Paramagnetic?

1s 1s σ1s

σ*1s

2s 2s σ2s

σ*2s

2p 2p

σ2p

π2p

σ*2p

π*2p

Ener

gy

O2 MO’s

Oxygen AO’s

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Molecular Geometry - chemed.tamu.educhemed.tamu.edu/chem101/pptnotes/geometry HO 6pp 13p.pdfValence Bond (VB) Theory (orbital overlap model) ... Dr. Williamson’s Molecular Geometry