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Molecular shapes. Balls and sticks. Learning objectives. Apply VSEPR to predict electronic geometry and shapes of simple molecules Distinguish between polar and nonpolar bonds in molecules Predict polarity of simple molecules from bond polarity and molecular shape. Roadmap to polarity. - PowerPoint PPT Presentation
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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