Molecular geometry

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Molecular Geometry

HS Ch i t R id Learning Series
Chemistry Rapid L
i
S i
Wayne Huang, PhD
Kelly Deters, PhD
Russell Dahl, PhD
Elizabeth James, PhD

www.RapidLearningCenter.com/
© Rapid Learning Inc. All rights reserved.

© Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com

1


Learning Objectives
By completing this tutorial you will learn…
Valence Shell Electron Pair
Repulsion Theory
p
y
Electronic and Molecular
Geometry

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Concept Map
Previous content

Chemistry
New content
Studies

Matter
One type is

Valence
Valance
Bond
Bond
Theory
Theory

1 bonding
theory is

Shown with

Lewis
Lewis
Structures
Structures

Compounds

Molecular
Geometry
Used to
determine

Electronic
Geometry
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2


Electronic and
Molecular
Geometry

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VSEPR Theory

V alence
S hell
E lectron
i
P air
Repulsion Theory
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3


What Does VSEPR Mean?
Bonds are made
of shared
electrons
(negatively
charged
subatomic
particles).

Negatively
charged things
repel each other.

Valence Sh ll El t
V l
Shell Electron P i
Pair

Geometry:
Bonds form as
far apart from
each other (and
other electrons)
as possible.

To predict
geometry

Repulsion
R
l i

VSEPR is a model used to predict molecular geometry based on
minimizing the electrostatic repulsion of valence electrons around a
central atom in a molecule.
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Definition: Electronic & Molecular
Geometry
Electronic Geometry – Uses the
VSEPR theory with the electron
y
regions around the central atom. An
electron bond is a bond (single,
double or triple…they all count as
one region).

Electron Geometry:
H2O = Tetrahedral
(with 2 electron lone pairs)

Molecular Geometry – Uses the
VSEPR theory with the atoms
bonded around the central atom
(spatial arrangement).
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Molecular Geometry:
H2O = Bent
(No electrons shown)

4


How to Count Electron Regions
Electronic geometry depends on electron regions
surrounding the central atom.
1
NH3 has 4
electron
regions.

2

H N H
3

H

Each Bond
Type

Electron
Region

Single Bond

4

1

H C

Double Bond

N

1

1

Triple Bond

1

Lone Pair

HCN has 2
electron
regions.

1

2
The triple bond
has only one
electron region.

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Chemical Formulas for Geometry
Each geometry has a “generic” chemical formula.
Central Atom

Lone Pairs on Central Atom

A X E
Two Types of Electron Regions:
E – “Lone Pair” Electron Region
X – “Bonded” Electron Region

Atoms Bonded to
Central Atom (“Ligand”)

e.g.

AX3E

X A X
X

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5


Linear Geometry
AX2

- Central Atom
- Electron Region

2 electron regions

Bonds 180° apart

Named: looks like a “line”.
Examples: CO2, BeH2
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Trigonal Planar Geometry
AX3

- Central Atom
- Electron Region

3 electron regions

Bonds 120° apart

Named: It’s a flat (“planar”) triangle.
Examples: BF3, C2H4
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6


Tetrahedron Geometry
AX4

- Central Atom

4 electron regions

- Electron Region

Bonds 109.5° apart
Named: If each plane (defined by 3 points) is covered, it’s a 4
(“tetra”) sided object (“hedron”).
Examples: CH4, SO2Cl2
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Trigonal Bipyramidal Geometry
AX5
5 electron regions

- Central Atom
- Electron Region

Bonds: Inside “Triangle” = 120°
Triangle
120
Between top/triangle/bottom = 90°
Named: If each plane (defined by 3 points) is covered, 2
pyramids with triangular bases are sitting base-to-base.
Examples: PCl5, AsF5
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7


Octahedron Geometry
AX6
6 electron regions

- Central Atom
- Electron Region

Bonds: 90
90°
Named: If each plane (defined by 3 points) is covered, it’s an
8 (“Octa”) sided object (“hedron”).
Examples: SF6, PF615/32

Determining Electronic Geometry
Electronic geometry is determined by electron regions.
Electron
regions

Angle between
regions

2

Linear

180°

3

Trigonal Planar

120°

4

Tetrahedral

109.5°

5

Trigonal Bipyramidal

90° and 120°

6

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Name of geometry

Octahedron

Picture

90°

Electronic Geometry Mnemonic: Linear(2) – Trigonal Planar(3) – Tetrahedral
(4) – Trigonal Bipyramidal(5) – Octahedron(6) = Long TriP To TriBe Overseas.


8


Determining Molecular Geometry
Molecular geometry is determined by # of atoms bonded to
the central atom (= #Electron Regions - #Lone Pairs).
Formula

Name of geometry

Angle between
bonded atoms

AX2

Linear

180°

AX3

Trigonal Planar

120°

AX4

Tetrahedral

109.5
109 5°

AX5


90° and 120°

AX6

Octahedron

Picture

90°

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Geometry with Lone Pairs
Use the formula to determine the electronic geometry.
Each atom bonded to the central atom (ligand)
counts as 1. Each lone pair on the central atom
counts as 1.
e.g. AX2E2
2 bonds & 2 lone pairs
= 4 electron regions
Electronic: Tetrahedral
Molecular: Bent
To determine molecular geometry, first determine
the electronic geometry and then remove lone pairs
and re-name the geometry.
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9


Bent Geometry with 1 Lone Pair
AX2E

E

Remove 1 “E”

Start with 3 electron regions (2X+E).
Trigonal Planar

Bent 120º

Named: It looks like a “bent” line.
Examples: SO2, O3
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Bent Geometry with 2 Lone Pairs
AX2E2
E

Remove 2 “E”s
E

Start with 4 electron regions (2X+2E).
Tetrahedron

Bent 109.5º

Named: It looks like a “bent” line.
Examples: H2O, SF2
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10


Trigonal Pyramidal Geometry
AX3E
E

Remove 1 “E”

Tetrahedron

Trigonal Pyramidal

Named: It looks like a pyramid with a triangular base.
Examples: NH3, SOCl2
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See-Saw Geometry
AX4E

E

Remove 1 “E”

Named: It looks like a see-saw.
Examples: SF4, O2XeF2

See-Saw

Note: This geometry has two
different ligand positions, axial
and equatorial. The lone pair at
equatorial would be less
crowded and more favorable.

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11


T-Shaped Geometry
AX3E2

E

Remove 2 “E”s
E

Named: It looks like a “T”.
Examples: ClF3, BrF3

T-Shaped

and equatorial. The lone pairs
at equatorial would be less

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Linear with 3 Lone Pairs
AX2E3

E

E

Remove 3 “E”s

E

Named: It looks like a “line”.
Examples: XeF2, I3-

Linear

and equatorial. The lone pairs
at equatorial would be less

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12


Square Pyramidal Geometry
AX5E
E

Remove 1 “E”

Octahedron

Square Pyramidal

Named: It looks like a pyramid with a square base.
Examples: BrF5, IF5
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Square Planar Geometry
AX4E2
E

Remove 2 “E”s
E

Octahedron

Square Planar

Named: It looks a flat (“planar”) square.
Examples: ICl4-, XeCl4
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13


Effect of Lone Pairs on Molecular Geometry
Both molecules have 4 electron regions:

H
H C H
H

H N H
H

These electrons
(lone pair) are not
being “controlled”
by another nucleus.

Lone pairs take up more space than a bonding pair—they distort
the bond angles slightly (push the bonding pairs closer).
Instead of being 109.5° angles (tetrahedron), the angles are 107.3°.

107.3°
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H N H
H

107.3°

107.3°

2D Drawing

3D Drawing

Determining Geometry with Lone Pairs
Formula

Name of geometry

AX2E

Bent

AX2E2

Bent

AX3E

Trigonal Pyramidal

AX4E

See-Saw

AX3E2

T-Shaped

AX2E3

Linear

AX5E

Square pyramidal

AX4E2

Picture

Square planar

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14


Determining Geometry - Example
Example: Give the electronic and molecular geometry for H2O.

Lone Pair 1

2 Bond

H O H
Bond 4

3 Lone Pair

Step 1: Draw the Lewis Structure.
Step 2: Determine # of electron regions (bonding and lone pair regions).
Step 3: Determine electronic geometry (based on # of electron regions).
4 electron regions = tetrahedral
Step 4: Determine molecular geometry. Count atoms bonded to and
lone pairs around the central atom. Ignore the lone pairs and adjust the
geometry (bond angles).
H2O: 2 bonded atoms and 2 lone pairs = bent molecular geometry.

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Note: This problem is commonly answered incorrectly as
it “looks” linear as it’s written above. However the lone
pairs make it bent!

Learning Summary

Valence Shell Electron
Pair Repulsion Theory
(VSEPR) is used to
determine geometry of
molecules (by
minimizing the electroni i i i th l t
pair repulsion).

Electronic geometry is
determined by electron
regions, while
molecular geometry is
determined by atoms
bonded t th
b d d to the central
t l
atom (bonding pairs).

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15


Congratulations
You have successfully completed
the tutorial

Molecular Geometry

Chemistry :: Biology :: Physics :: Math

What’s N t
Wh t’ Next …

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Molecular geometry

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