VSEPR What shape are your molecules in?. Background you need… Lewis structures How many bonds do each element make? What can expand? Bonding (covalent)

VSEPRVSEPR

What shape are your molecules in?What shape are your molecules in?

Background you need… Background you need…

Lewis structuresLewis structures How many bonds do each element make?How many bonds do each element make? What can expand?What can expand?

Bonding (covalent)Bonding (covalent) PolarityPolarity

Electronegativity and determining bond typeElectronegativity and determining bond type Resonance v. IsomersResonance v. Isomers Formal chargeFormal charge

Let’s review Let’s review now…..now…..

Lewis StructuresLewis Structures

Remember that Lewis structures want a Remember that Lewis structures want a full outer shellfull outer shell

Remember that for a given Lewis Remember that for a given Lewis structure, the number of electrons around structure, the number of electrons around the atoms must equal the total number of the atoms must equal the total number of electrons individually assigned. electrons individually assigned. Ex: C has 4, H has 1, so CHEx: C has 4, H has 1, so CH44 must have 8 must have 8

total total

IsomersIsomers

Same formula, different arrangement of Same formula, different arrangement of atomsatoms

Physically break bonds and MOVE Physically break bonds and MOVE atomsatoms

Resonance StructuresResonance Structures

Have the same Have the same alignment of alignment of atoms, but atoms, but different bonding different bonding (electrons ONLY (electrons ONLY are moved, both are moved, both in bonds and in bonds and lone pairs)lone pairs)

Determining formal Determining formal chargecharge

Formal charge can be determined by:

Normal number of electrons in outer shell

-[(1/2 the number of bonded electrons)

+ lone electrons]

_____________________________________= formal charge

Example: N in NH4

FC =5- [(1/2 of 8)+ 0]= +1

Formal charge and Formal charge and stabilitystability

The most “happy” molecules tend to have The most “happy” molecules tend to have no formal chargesno formal charges

However, molecules may be “happy” if However, molecules may be “happy” if they have not NET charge on them (if they have not NET charge on them (if there is 1+ and 1-, so a net of +1 + (-1)=0)there is 1+ and 1-, so a net of +1 + (-1)=0)

Resonance structures that are the best Resonance structures that are the best have a minimal formal charge and a full have a minimal formal charge and a full octet around each atomoctet around each atom

What is VSEPR?What is VSEPR?

ValenceValence ShellShell ElectronElectron PairPair RepulsionRepulsion TheoryTheory

Why?Why?

The shape of molecules influences their The shape of molecules influences their characteristics:characteristics: Things like polarity which influence things Things like polarity which influence things

likelike boiling point, melting point, which dictate their boiling point, melting point, which dictate their

nature (solid, liquid or gas at room temperature)nature (solid, liquid or gas at room temperature)

The parent geometries: all others come from these

Steric NumberSteric Number

The number of “things” sprouting off of an The number of “things” sprouting off of an atomatom These can be eitherThese can be either

Bonds Bonds Of any order (1, 2, or 3)Of any order (1, 2, or 3)

OrOr

Lone pairs of electronsLone pairs of electrons

Steric Number ExamplesSteric Number Examples Ex #1: CHEx #1: CH44

There are 4 H’s branching There are 4 H’s branching off , so the steric number off , so the steric number is 4is 4

SN=4SN=4

Ex #2: HEx #2: H22OO SN= 4SN= 4

Explain why Explain why

Ex #3: COEx #3: CO22

SN= 2SN= 2 Explain whyExplain why

General FormulasGeneral Formulas All molecules with a shared general All molecules with a shared general

formula have a shared geometryformula have a shared geometry we use them to help note shapewe use them to help note shape

Formulas are typically written with A’s, Formulas are typically written with A’s, X’s, and E’s X’s, and E’s

The letters stand for:The letters stand for: A= the central atomA= the central atom X *= the number of atoms attached to the central X *= the number of atoms attached to the central

atomatom E= the number of lone pairs of electrons attached E= the number of lone pairs of electrons attached

to the central atomto the central atom

*Some sources use A’s, B’s, and E’s*Some sources use A’s, B’s, and E’s

General Formula ExamplesGeneral Formula Examples Ex #1: CHEx #1: CH44

AXAX44

Ex #2: HEx #2: H22OO AXAX22EE22

Ex #3: COEx #3: CO22

AXAX22

LinearLinear

AXAX22

Trigonal planarTrigonal planar

AXAX33

TetrahedralTetrahedral

AXAX44

Pyramidal (Trigonal or Pyramidal (Trigonal or tetrahedral)tetrahedral)

Tetrahedral parent Tetrahedral parent shapeshape

1 lone pair of 1 lone pair of electronselectrons

AXAX33EE

BentBent

Tetrahedral Tetrahedral parent shapeparent shape


AXAX22EE22

When determining When determining polarity it is polarity it is important to look at important to look at the dipole moments- do the dipole moments- do they cancel out?they cancel out?

Trigonal bipyramidalTrigonal bipyramidal

AXAX55

Seesaw Seesaw a.k.a.a.k.a. Teeter- Teeter-tottertotter

Trigonal bipyramidal Trigonal bipyramidal parent shapeparent shape


AXAX44EE

T-shapedT-shaped



AXAX33EE22

LinearLinear



AXAX22EE33

OctahedralOctahedral

AXAX66

Square pyramidalSquare pyramidal

Octahedral parent Octahedral parent shapeshape


AXAX55EE

Square planarSquare planar



AXAX44EE22

T-shapedT-shaped



AXAX33EE33

Summary of shapesSummary of shapes

ID these VSEPR shapes…ID these VSEPR shapes…

Sweet drill and practice Sweet drill and practice web siteweb site

Given generic shapes to ID:Given generic shapes to ID: http://www.chemistry-drills.com/VSEPR-1.php?http://www.chemistry-drills.com/VSEPR-1.php?

q=1q=1

Given molecules to draw out:Given molecules to draw out: Basic: http://www.chemistry-drills.com/VSEPR-Basic: http://www.chemistry-drills.com/VSEPR-

1.php?q=21.php?q=2 Advanced: Advanced:

http://www.chemistry-drills.com/VSEPR-1.php?http://www.chemistry-drills.com/VSEPR-1.php?q=3q=3

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VSEPR What shape are your molecules in?. Background you need… Lewis structures How many bonds do each element make? What can expand? Bonding (covalent)