Molecular shapes Balls and sticks. Learning objectives Apply VSEPR to predict electronic geometry and shapes of simple molecules Distinguish between.

Molecular shapesMolecular shapes

Balls and sticksBalls and sticks

Learning objectivesLearning objectives

Apply VSEPR to predict electronic geometry Apply VSEPR to predict electronic geometry and shapes of simple moleculesand shapes of simple molecules

Distinguish between polar and nonpolar Distinguish between polar and nonpolar bonds in moleculesbonds in molecules

Predict polarity of simple molecules from Predict polarity of simple molecules from bond polarity and molecular shapebond polarity and molecular shape

Roadmap to polarityRoadmap to polarity

Establish skeleton of Establish skeleton of moleculemolecule

Determine Lewis dot Determine Lewis dot structure using S = N – Astructure using S = N – A

Determine electronic Determine electronic geometry using VSEPRgeometry using VSEPR

Identify molecular Identify molecular geometry from moleculargeometry from molecular

Count number of polar Count number of polar bondsbonds

Perform polarity analysisPerform polarity analysis

Valence shell electron pair repulsionValence shell electron pair repulsion

Lewis dot structure provides 2D sketch of Lewis dot structure provides 2D sketch of the distribution of the valence electrons the distribution of the valence electrons among bonds between atoms and lone among bonds between atoms and lone pairs; it provides no information about pairs; it provides no information about molecular shapemolecular shape

First approach to this problem is to consider First approach to this problem is to consider repulsion between groups of electrons repulsion between groups of electrons (charge clouds)(charge clouds)

Electron groups (clouds) minimize Electron groups (clouds) minimize potential energypotential energy

Valence shell electron pair Valence shell electron pair repulsion (VSEPR)repulsion (VSEPR) Identify all groups of charge: non-Identify all groups of charge: non-

bonding pairs or bonds (multiples bonding pairs or bonds (multiples count as one)count as one)

Bonded atoms – single, double Bonded atoms – single, double or triple count as 1or triple count as 1

Lone pairs count as 1Lone pairs count as 1 Distribute them about central Distribute them about central

atom to minimize potential energy atom to minimize potential energy (maximum separation)(maximum separation)

Choices are limited Choices are limited

Groups of charge range from 2 – 6Groups of charge range from 2 – 6 Only one electronic geometry in each caseOnly one electronic geometry in each case More than one molecular shape follows from More than one molecular shape follows from

electronic geometry depending on number of lone electronic geometry depending on number of lone pairspairs

One surprise: the lone pairs occupy more space One surprise: the lone pairs occupy more space than the bonded atoms (with very few exceptions)than the bonded atoms (with very few exceptions) Manifested in bond angles (examples follow)Manifested in bond angles (examples follow) Molecular shape selection (particularly in trigonal Molecular shape selection (particularly in trigonal

bipyramid)bipyramid)

Total number of groups dictates Total number of groups dictates electronicelectronic geometry geometry

Octet rule:Octet rule: Two – linearTwo – linear Three – trigonal planarThree – trigonal planar Four – tetrahedralFour – tetrahedral

Additional possibilities (expand octet):Additional possibilities (expand octet): Five – trigonal bipyramidalFive – trigonal bipyramidal Six - octahedralSix - octahedral

Stage 3: Molecular shape:Stage 3: Molecular shape:

1. Establish electronic structure using Lewis dot

model

2. Determine electronic geometry using VSEPR

model

3. Determine molecular shape from electronic

geometry

4. Determine molecule polarity using symmetry

model

What you get from electronic What you get from electronic geometry considering geometry considering atoms atoms onlyonly

Same tetrahedral Same tetrahedral electronicelectronic geometry – different geometry – different molecularmolecular shape shape

Two groups: linearTwo groups: linear

Except for BeHExcept for BeH22, all cases with two groups , all cases with two groups

involve multiple bondsinvolve multiple bonds

Three groups: trigonal planarThree groups: trigonal planar

Two possibilities for central atoms with Two possibilities for central atoms with complete octets:complete octets: Trigonal planar (HTrigonal planar (H22CO)CO)

Bent (SOBent (SO22))

BClBCl33 provides example of trigonal planar provides example of trigonal planar

with three single bondswith three single bonds B is satisfied with 6 electronsB is satisfied with 6 electrons

Four groups: tetrahedralFour groups: tetrahedral

Three possibilities:Three possibilities: No lone pairs (CHNo lone pairs (CH44) - tetrahedral) - tetrahedral

One lone pair (NHOne lone pair (NH33) – trigonal pyramid) – trigonal pyramid

Two lone pairs (HTwo lone pairs (H22O) – bentO) – bent

Note:Note:• H-N-H angle 107°H-N-H angle 107°• H-O-H angle 104.5°H-O-H angle 104.5°• Tetrahedral angle 109.5°Tetrahedral angle 109.5°

Representations of the tetrahedronRepresentations of the tetrahedron

Groups of charge

Lone electron pairs

Electronic geometry

Molecular shape

2 0 Linear Linear

3 0 Trigonal planar Trigonal planar

3 1 Trigonal planar Bent

4 0 Tetrahedral Tetrahedral

4 1 TetrahedralTrigonal pyramid

4 2 Tetrahedral Bent

Important properties related to Important properties related to polaritypolarity

Solubility: polar molecules dissolve in polar Solubility: polar molecules dissolve in polar solvents; nonpolar molecules dissolve in nonpolar solvents; nonpolar molecules dissolve in nonpolar solventssolvents Oil (nonpolar) and water (polar) don’t mixOil (nonpolar) and water (polar) don’t mix Ammonia (polar) dissolves in waterAmmonia (polar) dissolves in water

Melting and boiling pointsMelting and boiling points Polar substances have high intermolecular forces:Polar substances have high intermolecular forces: Melting and boiling points are much higher than with Melting and boiling points are much higher than with

nonpolar substances (Hnonpolar substances (H22O is a liquid, COO is a liquid, CO22 is a gas) is a gas)

Roadmap to polarityRoadmap to polarity

Establish skeleton of Establish skeleton of moleculemolecule

Determine Lewis dot Determine Lewis dot structure using S = N – Astructure using S = N – A

Determine electronic Determine electronic geometry using VSEPRgeometry using VSEPR

Identify molecular Identify molecular geometry from moleculargeometry from molecular

Count number of polar Count number of polar bondsbonds

Perform polarity analysisPerform polarity analysis

Polar bonds and polar moleculesPolar bonds and polar molecules

Not all molecules Not all molecules containing polar bonds containing polar bonds will themselves be polar.will themselves be polar.

Need to examine the Need to examine the molecular shapemolecular shape

Ask the question:Ask the question: Do the individual bond Do the individual bond

polarities cancel out?polarities cancel out? If so, non polar. If not, If so, non polar. If not,

polar.polar.

Consider some examplesConsider some examples

In COIn CO22 (linear molecule) the two polar bonds (linear molecule) the two polar bonds

oppose each other exactlyoppose each other exactly In chemical tug-o-war there is stalemateIn chemical tug-o-war there is stalemate

The most important polar moleculeThe most important polar molecule

In BFIn BF33 the three bonds cancel out – tug of the three bonds cancel out – tug of

war stalematewar stalemate In HIn H22O (bent) the polar bonds do not directly O (bent) the polar bonds do not directly

oppose – no stalemateoppose – no stalemate Lone pair also adds some componentLone pair also adds some component Overall net polarityOverall net polarity

Consequence of polarity: HConsequence of polarity: H22O is a liquid, O is a liquid,

COCO22 is a gas is a gas

Symmetry and polaritySymmetry and polarity

If the molecule “looks” If the molecule “looks” symmetrical it will be symmetrical it will be nonpolarnonpolar

If the molecule “looks” If the molecule “looks” non-symmetrical it will non-symmetrical it will be polarbe polar

Rules of thumb for evaluation of Rules of thumb for evaluation of polaritypolarity

Presence of one lone pair of electronsPresence of one lone pair of electrons Only one polar bondOnly one polar bond

Always polar moleculesAlways polar molecules

Two or more polar bondsTwo or more polar bonds Do polar bonds perfectly oppose?Do polar bonds perfectly oppose?

If no, polar moleculeIf no, polar molecule

Two bondsTwo bonds

Equal bonds oppose Equal bonds oppose (linear)(linear) Nonpolar (CONonpolar (CO22))

Unequal bonds oppose Unequal bonds oppose (linear)(linear) Polar (HCN)Polar (HCN)

Equal bonds do not Equal bonds do not oppose (bent)oppose (bent) Polar (HPolar (H22O)O)

Three bondsThree bonds

Equal bonds oppose in Equal bonds oppose in trigonal planar trigonal planar arrangementarrangement NonpolarNonpolar

Unequal bonds in Unequal bonds in trigonal planar trigonal planar arrangementarrangement Polar Polar