Molecular shapesMolecular shapes

Balls and sticksBalls and sticks

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

minimize potential energy (maximum minimize potential energy (maximum separation)separation)

This specifies the This specifies the electronicelectronic geometry also geometry also sometimes called sometimes called molecular molecular geometrygeometry

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)

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

S OO

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

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

Polar or non-polar? That is the Polar or non-polar? That is the question.question.

A molecule is polar if the centers of positive A molecule is polar if the centers of positive and negative charge do not coincide.and negative charge do not coincide.

How do we determine this?How do we determine this? Rigorous approach needs consideration of Rigorous approach needs consideration of

symmetry and mathematical calculationssymmetry and mathematical calculations Approximate approach considers arrangements Approximate approach considers arrangements

of bondsof bonds

How many polar bonds?How many polar bonds?

Bond is polar if electronegativity difference Bond is polar if electronegativity difference greater than 0.4greater than 0.4

Zero bonds: always nonpolarZero bonds: always nonpolar One bond: always polarOne bond: always polar Two or more bonds: may or may not…Two or more bonds: may or may not…

Consider the molecular shapeConsider the molecular shape Do individual bond polarities cancel?Do individual bond polarities cancel? If yes, nonpolar. If no, polarIf yes, nonpolar. If no, polar

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

Gets more complicatedGets more complicated

Planar or pyramidal?Planar or pyramidal? Depends on number of groups of chargeDepends on number of groups of charge BClBCl33 is trigonal planar – nonpolar is trigonal planar – nonpolar

NClNCl33 is trigonal pyramidal – polar is trigonal pyramidal – polar

Four bonds works better with modelsFour bonds works better with models

Roadmap to polarityRoadmap to polarity

Establish skeleton of moleculeEstablish skeleton of molecule Determine Lewis dot structure using S = N – ADetermine Lewis dot structure using S = N – A Determine electronic geometry using VSEPRDetermine electronic geometry using VSEPR Identify molecular geometry from molecularIdentify molecular geometry from molecular Count number of polar bondsCount number of polar bonds Perform polarity analysis using rules described Perform polarity analysis using rules described

aboveabove

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)