Electron Pair & Molecular Geometry

Introduction:

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1) The valence – shell electron – pair repulsion model (VSEPR) can predict the shape of molecules and ions by proposing that valence – shell electron pairs are arranged about each atom such that these pairs are as far away from one another as possible to minimize electron – pair repulsions.

2) Electron pair geometry is the shape that is obtained by assuming the electron pairs are groups around the central atom.

a. There are only five types of electron pair geometry:

i. Linear – two groups around central atom

ii. Trigonal planar – three groups around central atom

iii. Tetrahedral – four groups around the central atom

iv. Trigonal bipyramidal – five groups around the central atom

v. Octahedral – six groups around the central atom

3) Molecular geometry is the shape that is obtained by assuming the electron pairs are absent around the central atom.

a. There are multiple types of molecular geometries depending on the number of bonding groups (or substituents) and the number of loan pairs.

b. The table below shows all possible molecular geometries.

4) Molecular polarity is dependent upon the generation of dipole moments due to differences in electronegativity between bonding substituents and the geometry of the resultant molecule.

a. Nonpolar molecules have no net dipole.

b. Polar molecules have net nonzero dipole moments.

Geometries Predicted by the VSEPR Model

Valence Electron Pairs

Groups Bonded to Central Atom‡

Nonbonding Electron Pairs on Central Atom

VSEPR* Formula

Approx. Bond Angles

Electron Pair Geometry

Molecular Geometry

Examples

2

2

0

AX2

180o

linear

linear

BeF2

3

3

0

AX3

120o

trigonal planar

trigonal planar

BCl3

3

2

1

AX2E

<120o

trigonal planar

angular, bent

SnCl2

4

4

0

AX4

109.5o

tetrahedral

tetrahedral

CH4

4

3

1

AX3E

<109.5o

tetrahedral

trigonal pyramidal

PH3

4

2

2

AX2E​2

<109.5o

tetrahedral

angular, bent

H2O

5

5

0

AX5

90o/120o

trigonal bipyramidal

trigonal bipyramidal

PI5

5

4

1

AX4E

>90o/>120o

trigonal bipyramidal

irregular tetrahedral

SCl4

5

3

2

AX3E2

<90o

trigonal bipyramidal

T – shaped

ClF3

5

2

3

AX2E3

180o

trigonal bipyramidal

linear

XeF2

6

6

0

AX6

90o

octahedral

octahedral

SF6

6

5

1

AX5E

>90o

octahedral

square pyramidal

BrF5

6

4

2

AX4E2

90o

octahedral

square planar

ICl4–

‡ A double or triple bond counts as a single group.

*A = central atom, X = substituent, E = electron pair

Net Dipoles of Predicted Molecular Geometries

VSEPR* Formula

Molecular Geometry

Net Dipole†

AX

Linear

Can be nonzero

AX2

Linear

Zero

AX2

Bent

Can be nonzero

AX3

Trigonal planar

Zero

AX3

Trigonal pyramidal

Can be nonzero

AX3

T – shaped

Can be nonzero

AX4

Tetrahedral

Zero

AX4

Square planar

Zero

AX4

Irregular tetrahedral

Can be nonzero

AX5

Trigonal bipyramidal

Zero

AX5

Square pyramidal

Can be nonzero

AX6

Octahedral

Zero

*A = central atom, X = substituent, E = electron pair

†Assuming all atoms are identical

Substances to include in the Table of Chemical and Physical Properties:

NONE

Procedure:

1) Draw out the appropriate Lewis structure for all of the molecules or ions in the two charts below.

2) Using the online molecule viewer () construct these molecules. This is for visualization purpose only. Please attach 2-3 molecules for after saving the picture files.

3) Based on lecture video and instruction provided here determine the following information about each:

a. Number of bonding electron pairs between the central atom and the substituents

b. Number of nonbonding electron pairs on the central atom

c. Electron pair geometry

d. Molecular geometry

e. Approximate bond angles between the central atom and its substituents

f. Polarity of the resultant molecule or ion

4) After drawing Lewis structure, scan and upload it within your lab report.

Discussion:

1) Determine all structures in your data table.

2) Discuss the factor(s) that govern(s) the polarity of molecules.

3) What is meant by hybrid orbitals? Give two examples of the process of orbital hybridization.

4) What is formal charge? Explain the formula and how this can assist at determining correct Lewis structures.

Molecules/Ions that Obey Octet Rule

Molecule/Ion

Lewis Structure

Atoms

Bonded to Central atom

Nonbonding Electron Pairs on Central Atom

Electron Pair Geometry

Molecular Geometry

Approx. Bond Angles

Polar or Nonpolar

CH4

image1.wmf

4

0

Tetrahedral

Tetrahedral

109.5o

Nonpolar

OF2

NH3

SF2

H3O+

BrF2+

CO32–

BF4–

BeCl2

Molecule/Ion

Lewis Structure

Groups Bonded to Central Atom

Nonbonding Electron Pairs on Central Atom

Electron Pair Geometry

Molecular Geometry

Approx. Bond Angles

Polar or Nonpolar

SO42–

image2.wmf

4

0

Tetrahedral

Tetrahedral

~109.5o

Polar

AsF5

BrF4–

XeF4

I3–

BrF3

SnF62–

PF5

ICl2–

Banerjee

General Chemistry-I Lab

Note: BeCl2 does not satisfy the octet.

Molecules/Ions that Do Not Obey Octet Rule

PAGE

3

General Chemistry – I Banerjee

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