pair repulsion theory valence shell electron

Valence Shell Electron Valence Shell Electron Pair Repulsion TheoryPair Repulsion Theory

Chapter 9 Part IChapter 9 Part I

Lewis StructuresLewis Structures

1. Shows how the valence 1. Shows how the valence electrons are arranged. electrons are arranged.

2. Each dot for each valence 2. Each dot for each valence electron. electron.

Lewis StructuresLewis Structures3. A stable compound has all 3. A stable compound has all

its atoms with a noble gas its atoms with a noble gas configuration. configuration.

4. Hydrogen follows the duet 4. Hydrogen follows the duet rule. (2 electrons)rule. (2 electrons)

Lewis StructuresLewis Structures

5. The rest follow the octet 5. The rest follow the octet rule. (8 electrons)rule. (8 electrons)

6. The bonding pair is the one 6. The bonding pair is the one between the symbols. between the symbols.

DRAWING LEWIS DRAWING LEWIS STRUCTURESSTRUCTURES

How to Draw ThemHow to Draw Them

1. Sum the number of valence 1. Sum the number of valence electrons for each element;  electrons for each element;  the number of valence the number of valence electrons is equal to its electrons is equal to its (Roman) group number.(Roman) group number.

How to Draw ThemHow to Draw Them 2. Determine which element is 2. Determine which element is

the central atom, usually the the central atom, usually the least electronegative.least electronegative.

*Fluorine would never be a *Fluorine would never be a central element*central element*

How to Draw ThemHow to Draw Them 3. Use a pair of electrons to form 3. Use a pair of electrons to form

a bond between each pair of a bond between each pair of elements. elements.

4. Arrange the rest of the 4. Arrange the rest of the valence electrons to fulfill the valence electrons to fulfill the octet ruleoctet rule for each element.for each element.

How to Draw ThemHow to Draw Them 5. If an element does not have 5. If an element does not have

an octet use a double or triple an octet use a double or triple bond.bond.

6. A line can be used instead of 6. A line can be used instead of dots to represent a pair of dots to represent a pair of electrons.electrons.

Exceptions to the Octet Exceptions to the Octet RuleRule

1. Hydrogen and boron do not achieve an octet.2. Hydrogen accepts only 2 electrons.3. Boron accepts only 6 electrons4. Third row and larger elements can exceed the octet (examples: S & P)5. When we must exceed the octet, the extra electrons go on the central atom.

How to Draw ThemHow to Draw ThemExamples:Examples:

CHCH44

C is the central atomC is the central atom

H is an exception to the H is an exception to the octet ruleoctet rule

8 valence electrons8 valence electrons

How to Draw ThemHow to Draw Them

C

HH

HHH

HH

How to Draw ThemHow to Draw ThemNHNH33

N is the central atomN is the central atom

8 valence electrons8 valence electrons

How to Draw ThemHow to Draw Them

N

HH

HH

2 e left over2 e left over

Lone pairLone pair

How to Draw ThemHow to Draw Them

CO2

C is central atomC is central atom

16 valence electrons16 valence electrons

How to Draw ThemHow to Draw Them

CC OOOO

C does not have an octetC does not have an octet

Need 4 more valence electrons Need 4 more valence electrons since C does not have an octetsince C does not have an octet

How to Draw ThemHow to Draw Them

CC OOOO

Form double bondForm double bond

How to Draw ThemHow to Draw Them

CC OOOO

C still needs 2 electronsC still needs 2 electrons

Take 2 electrons from Take 2 electrons from other Oother O

How to Draw ThemHow to Draw Them

CC OOOO

Find the ErrorsFind the Errors •N has only 5

valence electrons•Si does not have

an octet.•H does not have a

duet.•N does not have

an octet.

Exceptions to the Octet Exceptions to the Octet RuleRule

•Beryllium and nitrogen are also exceptions.

•Odd e` compounds: A few are stable (NO,NO2, & ClO2)

ResonanceResonance-Refers to bonding in molecules or ions that can't -Refers to bonding in molecules or ions that can't be correctly represented by a single Lewis be correctly represented by a single Lewis structurestructure

-Hybrid structures are formed that are an -Hybrid structures are formed that are an average of all possible Lewis structuresaverage of all possible Lewis structures

Example: Ozone, OExample: Ozone, O33

O O O O O O O O O O O O

VSEPRVSEPR-Lewis structures tell us how the atoms are connected to each other. -They don’t tell us anything about shape. -VSEPR: Predicts 3D geometry of molecules.-Valence shell - outside electrons.

VSEPRVSEPR

-Electron Pair repulsion - electron pairs try to get as far away as possible. Goal is to minimize electron pair repulsions.

-Based on the number of pairs of valence electrons both bonded and lone pairs.

VSEPRVSEPRRules

1. Draw the Lewis structure2. Count # of electrons on the central

atom (paired & lone)3. Multiple bonds count as one4. Using this # determine the

geometry and shape.

LINEARLINEARLone Pairs: 0 Bonding Pairs: 2Lone Pairs: 0 Bonding Pairs: 2

Angle: 180Angle: 180ºº

Trigonal PlanarTrigonal PlanarBonding Pairs: 3 Lone Pairs: 0Bonding Pairs: 3 Lone Pairs: 0

Angle: 120Angle: 120ºº

Trigonal PlanarTrigonal PlanarBonding Pairs: 2 Lone Pairs: 1Bonding Pairs: 2 Lone Pairs: 1

Angle: 120Angle: 120º º Shape: Bent Shape: Bent

TetrahedralTetrahedralBonding Pairs: 4 Lone Pairs: 0Bonding Pairs: 4 Lone Pairs: 0

Angle: 109Angle: 109ºº

TetrahedralTetrahedralBonding Pairs: 3 Lone Pairs: 1

Angle: 109º Shape: Trigonal Pyramidal

TetrahedralTetrahedralBonding Pairs: 2 Lone Pairs: 2

Angle: 109º Shape: Bent

Trigonal BiPyramidalTrigonal BiPyramidalBonding Pairs: 5 Lone Pairs: 0

Angle: 120º ,90º

Trigonal BiPyramidalTrigonal BiPyramidalBonding Pairs: 4 Lone Pairs: 1Bonding Pairs: 4 Lone Pairs: 1

Angle: 120Angle: 120º º ,90,90º º Shape: SeesawShape: Seesaw

Trigonal BiPyramidalTrigonal BiPyramidalBonding Pairs: 3 Lone Pairs: 2Bonding Pairs: 3 Lone Pairs: 2

Angle: 120Angle: 120º º ,90,90º º Shape: T-ShapedShape: T-Shaped

Trigonal BiPyramidalTrigonal BiPyramidalBonding Pairs: 2 Lone Pairs: 3Bonding Pairs: 2 Lone Pairs: 3

Angle: 120Angle: 120º º ,90,90º º Shape: LinearShape: Linear

OctahedralOctahedralBonding Pairs: 6 Lone Pairs: 0Bonding Pairs: 6 Lone Pairs: 0

Angle: 90Angle: 90º º

OctahedralOctahedralBonding Pairs: 5 Lone Pairs: 1Bonding Pairs: 5 Lone Pairs: 1

Angle: 90Angle: 90º º Shape: Square PyramidalShape: Square Pyramidal

OctahedralOctahedralBonding Pairs: 4 Lone Pairs: 2Bonding Pairs: 4 Lone Pairs: 2

Angle: 90Angle: 90º º Shape: Square PlanarShape: Square Planar

VSEPR ExamplesVSEPR Examples

VSEPRVSEPR

CC

HH

HHHH

HHCHCH44

Structural FormulaStructural Formula

4 bonding 4 bonding electronselectrons

VSEPRVSEPRUse chartUse chart

4 bonding 0 lone around 4 bonding 0 lone around central elementcentral element

Geometry: tetrahedralGeometry: tetrahedral

Shape: tetrahedralShape: tetrahedral

VSEPRVSEPR Examples: Examples:

XeF XeF44

IF IF55--

OF OF22

VSEPRVSEPR

XeXe

FF

FFFF

FF

2 pairs of electrons left over2 pairs of electrons left over

Place extra on XePlace extra on Xe

Geometry: Geometry: OctahedralOctahedral

Shape: Square Shape: Square PlanarPlanar

VSEPRVSEPR

II

FF

FFFF

FF

1 pair left overPlace on IPlace on I

FF

Geometry: Geometry: OctahedralOctahedral

Shape: Square Shape: Square PyramidalPyramidal

VSEPRVSEPR

OO FFFF

Geometry: tetrahedralGeometry: tetrahedral Shape: bentShape: bent

Bond PolarityBond Polarity

Polar BondsPolar Bonds 1. When the atoms in a bond 1. When the atoms in a bond

are the same, the electrons are the same, the electrons are shared equally.are shared equally.

This is a nonpolar covalent This is a nonpolar covalent bond.bond.

Polar BondsPolar Bonds 2. When two different atoms 2. When two different atoms

are connected, the electrons are connected, the electrons may not be shared equally.may not be shared equally.

This is a polar covalent bond. This is a polar covalent bond.

Polar BondsPolar Bonds 3. Use electronegativity to 3. Use electronegativity to

determine which type of bond.determine which type of bond.

The bigger the electronegativity The bigger the electronegativity difference the more polar the difference the more polar the bond. bond.

Polar BondsPolar Bonds0.0 - 0.5 Covalent nonpolar0.0 - 0.5 Covalent nonpolar

0.5 - 1.0 Covalent moderately 0.5 - 1.0 Covalent moderately polarpolar

1.0 -2.0 Covalent polar1.0 -2.0 Covalent polar

>2.0 Ionic>2.0 Ionic

How to Show a Polar BondHow to Show a Polar Bond

1. Isn’t a whole charge just 1. Isn’t a whole charge just

a partial charge a partial charge

2. 2. + means a partially positive + means a partially positive chargecharge

- means a partially negative - means a partially negative chargecharge

How to Show a Polar BondHow to Show a Polar Bond

The Cl pulls harder on the electrons

HH ClCl + -

The electrons spend more The electrons spend more time near the Cl time near the Cl

Determining PolarityDetermining Polarity 1. If composed of one kind of 1. If composed of one kind of

atom it’s nonpolaratom it’s nonpolar

2. Determined by shape of the 2. Determined by shape of the moleculemolecule

Determining PolarityDetermining Polarity

3. Symmetrical: nonpolar A. Trigonal Planar

B. Tetrahedral

C. Trigonal Bipyramidal D. Octahedral E. Square Planar

Determining PolarityDetermining Polarity4. Unsymmetrical: Polar A. Bent

B. Trigonal Pyramidal C. T-shaped D. Square Pyramidal