Molecular shapesMolecular shapes

A simple matter of balls and sticksA simple matter of balls and sticks

Learning objectivesLearning objectives

Describe underlying principles that govern Describe underlying principles that govern theories of molecular shapestheories of molecular shapes

Use Lewis dot diagrams to predict shapes of Use Lewis dot diagrams to predict shapes of molecules using VSEPRmolecules using VSEPR

Valence shell electron pair repulsionValence shell electron pair repulsion

In order to understand properties like In order to understand properties like polarity, we need to predict molecular polarity, we need to predict molecular shapesshapes

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 the pairs; it provides no information about the shape of the moleculeshape of the molecule

A hierarchy of models A hierarchy of models

VSEPRVSEPR Consider the problem in terms of electrostatic repulsion Consider the problem in terms of electrostatic repulsion

between groups of electrons (charge clouds, domains)between groups of electrons (charge clouds, domains)

Valence bond theoryValence bond theory Acknowledges the role of orbitals in covalent bondingAcknowledges the role of orbitals in covalent bonding

Molecular orbital (MO) theory (the “real” thing)Molecular orbital (MO) theory (the “real” thing) Accommodates delocalization of electrons - explains Accommodates delocalization of electrons - explains

optical and magnetic propertiesoptical and magnetic properties

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

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

pairs and bonds (multiples count as one)pairs and bonds (multiples count as one) Distribute them about the central atom to Distribute them about the central atom to

minimize potential energy (maximum separation minimize potential energy (maximum separation of the groups)of the groups)

This specifies the This specifies the electronicelectronic geometry (also geometry (also known as known as electron domain electron domain geometry or geometry or sometimes confusingly as sometimes confusingly as molecularmolecular geometry) geometry)

Choices are limited Choices are limited

Groups (domains) of charge range from 2 – 6Groups (domains) of charge range from 2 – 6 Only one electronic geometry in each caseOnly one electronic geometry in each case However, more than one molecular shape follows However, more than one molecular shape follows

from electronic geometry depending on number of from electronic geometry depending on number of lone pairslone pairs

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)

Two groups: linearTwo groups: linear

Except for BeHExcept for BeH2 2 (Be violates octet rule), all cases (Be violates octet rule), all cases

with two groups involve multiple bondswith two groups involve multiple bonds

Three groups: trigonal planarThree groups: trigonal planar

Two possibilities for Two possibilities for central atoms with central atoms with complete octets:complete octets: Trigonal planar (HTrigonal planar (H22CO)CO) Bent (SOBent (SO22))

BClBCl33 provides example provides example of trigonal planar with of trigonal planar with three single bondsthree single bonds B is satisfied with 6 B is satisfied with 6

electrons – violates electrons – violates octet ruleoctet rule

Four groups: tetrahedralFour groups: tetrahedral

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

tetrahedraltetrahedral One lone pair (NHOne lone pair (NH33) – ) –

trigonal pyramidtrigonal pyramid Two lone pairs (HTwo lone pairs (H22O) – O) –

bentbent Lone pairs need space:Lone pairs need space:

• 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

Five groups of charge: trigonal Five groups of charge: trigonal bipyramid – most variationsbipyramid – most variations

Two different positions:Two different positions: Three equatorialThree equatorial Two axialTwo axial

Equatorial positions are lower energy:Equatorial positions are lower energy: Lone pairs require occupy these locations preferentiallyLone pairs require occupy these locations preferentially

Five bonds, no lone pairsFive bonds, no lone pairs

Four bonds, one lone pairFour bonds, one lone pair

Lone pair dictates geometry: equatorial position Lone pair dictates geometry: equatorial position has lower energy than axialhas lower energy than axial

Three bonds, two lone pairsThree bonds, two lone pairs

Both lone pairs occupy equatorial positions – Both lone pairs occupy equatorial positions – lower energy than in axiallower energy than in axial

Two bonds, three lone pairsTwo bonds, three lone pairs

The trend continues: all equatorial positions filled – The trend continues: all equatorial positions filled – lowest energylowest energy

Octahedron has six identical Octahedron has six identical positions and high symmetrypositions and high symmetry

No lone pairsNo lone pairs

High symmetryHigh symmetry

One lone pairOne lone pair

All positions are equally probableAll positions are equally probable Symmetry reducedSymmetry reduced

Two lone pairsTwo lone pairs Minimum energy has axial symmetry, lone pairs lie Minimum energy has axial symmetry, lone pairs lie

along straight linealong straight line

Molecules with multiple centersMolecules with multiple centers

A central atom is any atom with more than one atom A central atom is any atom with more than one atom bonded to itbonded to it

Perform exercise individually for each atomPerform exercise individually for each atom Electronic geometry and molecular shape will refer only to Electronic geometry and molecular shape will refer only to

the atoms/lone pairs immediately attached to that atomthe atoms/lone pairs immediately attached to that atom

Taking it to the next level: Taking it to the next level: acknowledging orbitalsacknowledging orbitals

VSEPR is quite successful in predicting VSEPR is quite successful in predicting molecular shapes based on the simplistic molecular shapes based on the simplistic Lewis dot approachLewis dot approach

But our understanding of the atom has the But our understanding of the atom has the electrons occupying atomic orbitalselectrons occupying atomic orbitals

How do we reconcile the observed shapes How do we reconcile the observed shapes of molecules with the atomic orbital picture of molecules with the atomic orbital picture of atomsof atoms