molecular geometry

High School Chemistry Rapid Learning Series - 17

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Molecular Geometry

HS Ch i t R id L i S i

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

HS Chemistry Rapid Learning Series

Wayne Huang, PhDKelly Deters, PhDRussell Dahl, PhD

Elizabeth James, PhD



Learning Objectives

Valence Shell Electron Pair Repulsion Theory

By completing this tutorial you will learn…

p y

Electronic and Molecular Geometry

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Concept Map

Chemistry

Studies

Previous content

New content

Matter

Compounds

One type is

ValanceBond

Theory

1 bonding theory is

ValenceBond

Theory

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MolecularGeometry

ElectronicGeometry

StructuresLewis

Structures

Shown with

Used to determineStructures

Lewis Structures



Electronic andElectronic and Molecular Geometry

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VSEPR Theory

ValenceShellElectronP i

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PairRepulsion Theory



What Does VSEPR Mean?

Bonds are made of shared Geometry:

Bonds form aselectrons (negatively

charged subatomic particles).

Negatively charged things

repel each other.

Bonds form as far apart from

each other (and other electrons)

as possible.

V l Sh ll El t P i R l i To predict

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Valence Shell Electron Pair Repulsion To predict geometry

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.

Definition: Electronic & Molecular Geometry

Electronic Geometry – Uses the VSEPR theory with the electron yregions 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)

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Molecular Geometry – Uses the VSEPR theory with the atoms bonded around the central atom (spatial arrangement).

Molecular Geometry:H2O = Bent

(No electrons shown)



How to Count Electron Regions

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Electronic geometry depends on electron regions surrounding the central atom.

3

21

4

N HHH

C NH

NH3 has 4electron regions.

HCN has 2

Each Bond Type

Electron Region

Single Bond 1

Double Bond 1

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1 2The triple bond has only one electron region.

C NHHCN has 2electron regions.

Triple Bond 1

Lone Pair 1

Chemical Formulas for Geometry

Lone Pairs on Central AtomCentral Atom

Each geometry has a “generic” chemical formula.

X EA

Lone Pairs on Central AtomCentral Atom

Atoms Bonded to Central Atom (“Ligand”)

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

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e.g. AX3E AXX

X



Linear GeometryAX2

2 electron regions

- Central Atom

- Electron Region2 electron regions

Bonds 180° apart

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Examples: CO2, BeH2

Named: looks like a “line”.

Trigonal Planar GeometryAX3

3 electron regions

- Central Atom

- Electron Region3 electron regions

Bonds 120° apart

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Examples: BF3, C2H4

Named: It’s a flat (“planar”) triangle.



Tetrahedron GeometryAX4

4 electron regions

- Central Atom

- Electron Region

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Bonds 109.5° apart

Examples: CH4, SO2Cl2

Named: If each plane (defined by 3 points) is covered, it’s a 4 (“tetra”) sided object (“hedron”).

Trigonal Bipyramidal GeometryAX5

5 electron regions- Central Atom

- Electron Region

Bonds: Inside “Triangle” = 120°

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Bonds: Inside Triangle 120Between top/triangle/bottom = 90°

Examples: PCl5, AsF5

Named: If each plane (defined by 3 points) is covered, 2 pyramids with triangular bases are sitting base-to-base.



Octahedron GeometryAX6

6 electron regions- Central Atom

- Electron Region

Bonds: 90°

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Bonds: 90

Examples: SF6, PF6-

Named: If each plane (defined by 3 points) is covered, it’s an 8 (“Octa”) sided object (“hedron”).

Determining Electronic GeometryElectronic geometry is determined by electron regions.

Electron regions

Name of geometry Angle between regions

Picture

Linear

Trigonal Planar

Tetrahedral

180°

120°

109.5°

2

3

4

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Trigonal Bipyramidal

Octahedron

90° and 120°

90°

5

6

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



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

Formula Name of geometry Angle between bonded atoms

Picture

Linear

Trigonal Planar

Tetrahedral

bonded atoms

180°

120°

109 5°

AX2

AX3

AX

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Tetrahedral


Octahedron

109.5

90° and 120°

90°

AX4

AX5

AX6

Geometry with Lone PairsUse 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. AX2E22 bonds & 2 lone pairs = 4 electron regionsElectronic: TetrahedralMolecular: Bent

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Molecular: Bent

To determine molecular geometry, first determine the electronic geometry and then remove lone pairs and re-name the geometry.



Bent Geometry with 1 Lone PairAX2E

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

Remove 1 “E”

Bent 120º

E

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Examples: SO2, O3

Named: It looks like a “bent” line.

Trigonal Planar

Bent Geometry with 2 Lone PairsAX2E2

E

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

Remove 2 “E”s

Bent 109.5º

E

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Examples: H2O, SF2

Named: It looks like a “bent” line.

Tetrahedron



Trigonal Pyramidal GeometryAX3E

E

Start with 4 electron regions (3X+E).Tetrahedron

Remove 1 “E”

Trigonal Pyramidal

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Examples: NH3, SOCl2

Named: It looks like a pyramid with a triangular base.

Tetrahedron

See-Saw GeometryAX4E

Start with 5 electron regions (4X+E).Trigonal Bipyramidal

Remove 1 “E”

See-Saw

E

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Examples: SF4, O2XeF2

Named: It looks like a 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.



T-Shaped GeometryAX3E2

Start with 5 electron regions (3X+2E).Trigonal Bipyramidal

Remove 2 “E”s

T-Shaped

EE

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Examples: ClF3, BrF3

Named: It looks like a “T”.


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

Linear with 3 Lone PairsAX2E3

Start with 5 electron regions (2X+3E).Trigonal Bipyramidal

Remove 3 “E”s

Linear

EE

E

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Examples: XeF2, I3-

Named: It looks like a “line”.


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



Square Pyramidal GeometryAX5E

E

Start with 6 electron regions (5X+E).Octahedron

Remove 1 “E”

Square Pyramidal

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Examples: BrF5, IF5

Named: It looks like a pyramid with a square base.

Octahedron

Square Planar GeometryAX4E2

E

Start with 6 electron regions (4X+2E).Octahedron

Remove 2 “E”s

Square Planar

E

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Examples: ICl4-, XeCl4

Named: It looks a flat (“planar”) square.

Octahedron



Effect of Lone Pairs on Molecular Geometry

N HHH

Both molecules have 4 electron regions:

These electronsN HH

HC HHH

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).

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Instead of being 109.5° angles (tetrahedron), the angles are 107.3°.

107.3°107.3°

N HHH 107.3°

2D Drawing 3D Drawing

Determining Geometry with Lone PairsFormula Name of geometry

Bent

Picture

AX2E

Bent

Trigonal Pyramidal

See-Saw

T-Shaped

AX2E2

AX3E

AX4E

AX3E2

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LinearAX2E3

Square pyramidalAX5E

Square planarAX4E2



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

O HH21 BondLone Pair

O HH

Step 1: Draw the Lewis Structure.

Step 2: Determine # of electron regions (bonding and lone pair regions).

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Step 3: Determine electronic geometry (based on # of electron regions).4 electron regions = tetrahedral

Lone PairBond

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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.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


(VSEPR) is used to determine geometry of

molecules (by i i i i th l t


(VSEPR) is used to determine geometry of

molecules (by i i i i th l t

Electronic geometry is determined by electron

regions, while molecular geometry is determined by atoms b d d t th t l

Electronic geometry is determined by electron

regions, while molecular geometry is determined by atoms b d d t th t l

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minimizing the electron-pair repulsion).

minimizing the electron-pair repulsion).

bonded to the central atom (bonding pairs).bonded to the central atom (bonding pairs).



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Molecular Geometry

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molecular geometry

Technology

electron geometry

electron bond

electron regionsangle

electron y regions

types of electron regions

learning objectives

x x1032 rapid learning

determineelectronic