WARM UP FEB 19 th 2013 Draw the Lewis structures for the following… 1.CO ₂ 2.IO ₄⁻ 3.HF 4.H...

WARM UP FEB 19th 2013

Draw the Lewis structures for the following…

1.CO₂2.IO₄⁻3.HF4.H₂O

The shape of a molecule may determine its The shape of a molecule may determine its properties and usesproperties and uses

Why the shape of a molecule Why the shape of a molecule is importantis important

Properties such as smell, taste, and Properties such as smell, taste, and proper targeting (of drugs) are all proper targeting (of drugs) are all possible because of the shapes of possible because of the shapes of

moleculesmolecules

Aspirin works because of its Aspirin works because of its shape!shape!

Prostaglandin which causes inflammation (swelling) is

produced by the COX-1 and COX-

2 enzymes

Aspirin can block the

substrate from bonding to the COX-1 or COX-2

enzyme thus preventing the production of prostaglandin

Lewis structures don’t give us a 3-Lewis structures don’t give us a 3-dimensional view of how the atoms are dimensional view of how the atoms are

bonded togetherbonded together

Determining the Shape of a Determining the Shape of a moleculemolecule

Would you have predicted this arrangement of atoms from

just seeing it’s Lewis structure?

The Lewis structure implies a cross shape with 90o

angles

By using the VSEPR TheoryBy using the VSEPR Theory

(pronounced Vess Purr)(pronounced Vess Purr)

So how do we find the So how do we find the shape of a molecule?shape of a molecule?

VValence alence SShell hell EElectron lectron PPair air RRepulsion epulsion TTheoryheory

Main Premise: Main Premise: Molecules will adopt a Molecules will adopt a shape that is lowest in energy by shape that is lowest in energy by minimizing the valence shell electron pair minimizing the valence shell electron pair repulsion (VSEPR) between adjacent atoms repulsion (VSEPR) between adjacent atoms

VSEPR TheoryVSEPR Theory

Atoms in a molecule try to spread out from one another

as much as possible to reduce the “like charge repulsion”

between their outer electrons

Huh???Huh???

C

H

H

H

Hmethane, CHmethane, CH44

But if you think in 3 dimensions, the But if you think in 3 dimensions, the hydrogens can actually get farther away from hydrogens can actually get farther away from each other and minimize adjacent electron each other and minimize adjacent electron cloud repulsionscloud repulsions

109.5°109.5°90°90°

C

H

H

H

HYou might think this is the You might think this is the

farthest that the hydrogens farthest that the hydrogens can get away from each can get away from each

other other

THE 5 MAIN VSEPR THE 5 MAIN VSEPR SHAPESSHAPES

These shapes minimize the like charge repulsion between adjacent

electron clouds

FROM LEWIS TO VSEPR SHAPEFROM LEWIS TO VSEPR SHAPE

1. Draw a Lewis structure

2. Count the number of “electron domains” around the central atom

-Each single, double and triple bond counts as ONE domain -Each lone pair counts as ONE domain

3. Use VSEPR Chart to determine the shape based on how many bonding and nonbonding domains are around the central atom

ELECTRON DOMAINSELECTRON DOMAINS

Regions in a molecule where there are high concentrations of electrons

Bonds = (bonding domains)

Lone pairs= (non-bonding domains)

This Lewis structure shows 2 bonding domains

and 2 non bonding domains

HOW MANY “DOMAINS” AROUND HOW MANY “DOMAINS” AROUND THE CENTRAL ATOM?THE CENTRAL ATOM?

4 around carbon

2 around each atom

3 around nitrogen

Remember: single, double and triple bonds count as

ONE domain

REMEMBER THE BIG PICTURE?REMEMBER THE BIG PICTURE?

Electron “domains” are all negatively charged so they want to spread out from each other as much

as possible to minimize like-charge-repulsion within a molecule

Doing this allows the molecule to be more stable (low energy)

THE VSEPR THE VSEPR CHARTCHART

You need to memorize this

LET’S LOOK AT SOME LET’S LOOK AT SOME EXAMPLESEXAMPLES

VSEPR EXAMPLE 1VSEPR EXAMPLE 1

• How many bonding and non-bonding electron domains are there around the central atom?• 2 bonding• 0 non-bonding


• Use the VSEPR chart…• Electron geometry (how the electron domains are

arranged around the central atom) is “linearlinear”• Molecular geometry (how the atoms bonded to the

central atom are arranged) is “linearlinear” also

2 bonding, 0 nonbonding



arranged around the central atom) is “trigonal planartrigonal planar”• Molecular geometry (how the atoms bonded to the central

atom are arranged) is “trigonal planartrigonal planar” also




arranged around the central atom) is “trigonal trigonal planarplanar”

• Molecular geometry (how the atoms bonded to the central atom are arranged) is “bentbent”




arranged around the central atom) is “tetrahedraltetrahedral”• Molecular geometry (how the atoms bonded to the

central atom are arranged) is “tetrahedraltetrahedral”





central atom are arranged) is “trigonal pyramidaltrigonal pyramidal”





central atom are arranged) is “bentbent”


Lone pairs (non-bonding domains) create a larger region of negative charge than

bonding domains and thus push the adjacently bonded atoms even farther

away from each other than normal

C

H

H

H

H NH H

HOH H

.. ....

109.5°109.5° 107°107° 104.5°104.5°

Lone pairs decrease the Lone pairs decrease the

expected bond angleexpected bond angle

FOR TETRAHEDRAL SHAPESFOR TETRAHEDRAL SHAPES

Number of lone pairs around central atom

0 1 2

Approximate bond angle

109.5 107 104.5

VSEPR NOTATIONVSEPR NOTATION

Also known as “AXEAXE” notation

It is just a shorthand way to communicate VSEPR information

EXAMPLES OF USINGEXAMPLES OF USING AXE NOTATION AXE NOTATION

AX3E

1

This subscript tells how many atoms are bonded to the central atom

This subscript tells

how many lone

pairs are on the

central atom

AX3E1 is always trigonal pyramidal


AX2E

2

This subscript tells how many atoms are bonded to the central atom

This subscript tells

how many lone

pairs are on the

central atom

AX2E2 is always bent


AX4This subscript tells how many atoms are bonded to the central atom

Don’t put the “E” if

there aren’t any

lone pairs

AX4 is always tetrahedral

FISHER PROJECTIONSFISHER PROJECTIONS

A way to make your Lewis structures indicate their three dimensional

VSEPR shape on paper

C

H

H

H

H NH H

H

OH H.. ..

..

H

CH H

HN

H HH

OH H

FISHER PROJECTIONSFISHER PROJECTIONS

Bonds in the plane of the paper are shown as lines

Bonds projecting in front of the plane of the paper are shown as triangles

Bonds projecting behind the plane of the paper are shown as stacked lines

WARM UP FEB 19 th 2013 Draw the Lewis structures for the following… 1.CO ₂ 2.IO ₄⁻ 3.HF 4.H...

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