The shapes of thingsThe shapes of things

Molecular shape determines Molecular shape determines propertiesproperties

Bonding determines shapeBonding determines shape

Learning objectivesLearning objectives

Write Lewis dot structures for simple Write Lewis dot structures for simple moleculesmolecules

Predict shape of simple moleculesPredict shape of simple molecules Predict polarity of simple moleculesPredict polarity of simple molecules

Covalent molecular compoundsCovalent molecular compounds

Covalent compounds are usually molecularCovalent compounds are usually molecular Bonds between atoms are covalentBonds between atoms are covalent Interactions between molecules are very weakInteractions between molecules are very weak Atoms in a covalent molecule don’t stack like Atoms in a covalent molecule don’t stack like

marblesmarbles Bonds have specific directionsBonds have specific directions

Molecules have specific shapesMolecules have specific shapes

Shape will depend onShape will depend on The number of atoms bonded to the central atomThe number of atoms bonded to the central atom The number of lone pairs around the central atomThe number of lone pairs around the central atom

Distinguish betweenDistinguish between Electronic geometry (molecular geometry)Electronic geometry (molecular geometry)

Consider atoms and lone pairsConsider atoms and lone pairs Molecular shapeMolecular shape

Consider atoms onlyConsider atoms only

Lewis dot structures: doing the dotsLewis dot structures: doing the dots Molecular structure in Molecular structure in

simplest terms: arrange simplest terms: arrange valence electrons as dots in a valence electrons as dots in a 2-dimensional figure2-dimensional figure Only valence electrons are Only valence electrons are

shownshown Electrons are either in:Electrons are either in:

bondsbonds lone pairs (stable molecules do lone pairs (stable molecules do

not contain unpaired electrons – not contain unpaired electrons – with very few exceptions)with very few exceptions)

Octet rule is guiding principle: Octet rule is guiding principle: each atom has 8 dots round it each atom has 8 dots round it (H has 2 dots)(H has 2 dots)

Lewis dot structures made easy: the Lewis dot structures made easy: the S = N –A machineS = N –A machine

Start with the skeleton of the moleculeStart with the skeleton of the molecule Least Least electronegative element is the central atomelectronegative element is the central atom

SS = = NN - - AA NN = total number of electrons required to fill octet for each = total number of electrons required to fill octet for each

atom in the molecule (8 for each element, except 2 for H atom in the molecule (8 for each element, except 2 for H and 6 for B)and 6 for B)

AA = total number of valence electrons = total number of valence electrons SS = total number of electrons in bonds = total number of electrons in bonds

We are given We are given NN and and AA; we need to find ; we need to find SS

Applying the rulesApplying the rules

Calculate Calculate NN for the molecule for the molecule Calculate Calculate AA (all the dots) (all the dots)

include charges for ions include charges for ions (add (add one for each –ve charge and one for each –ve charge and subtract one for each +ve charge)subtract one for each +ve charge)

Determine Determine SS (no of dots in (no of dots in bonds)bonds) ((SS = = NN – – A)A)

Satisfy all octets and create Satisfy all octets and create number of bonds dictated by number of bonds dictated by SS (may be multiple bonds) (may be multiple bonds)

NFNF33

NN = 8(N) + 3 x 8(F) = 32 = 8(N) + 3 x 8(F) = 32 AA = 5(N) + 3 x 7(F) = 26 = 5(N) + 3 x 7(F) = 26 SS = 32 – 26 = = 32 – 26 = 66

N FFF

N FFF

Two tests for dot structuresTwo tests for dot structures

Is the number of dots in the molecule equal to Is the number of dots in the molecule equal to the number of valence electrons?the number of valence electrons?

Are all the octets satisfied?Are all the octets satisfied? If both yes structure is validIf both yes structure is valid If either no then back to the drawing boardIf either no then back to the drawing board

Electronic geometryElectronic geometry

Identify central Identify central atom. Many atom. Many molecules have more molecules have more than one.than one. Central atom has Central atom has

more than one atom more than one atom bonded to itbonded to it

Methanol has two central atomsMethanol has two central atoms

O is one central atom – O is one central atom – bonded to H and Cbonded to H and C

C is another central C is another central atom – bonded to O, H, atom – bonded to O, H, H and HH and H

Consider geometry Consider geometry around each one around each one separatelyseparately

HH

H

O

H

Counting regions of chargeCounting regions of charge

Count only atoms and lone pairs immediately Count only atoms and lone pairs immediately bonded to central atombonded to central atom

Count the regions of electronsCount the regions of electrons Bonds – single, double or triple count as 1Bonds – single, double or triple count as 1 Lone pairs count as 1Lone pairs count as 1

Number will be between 2 and 4 for molecules Number will be between 2 and 4 for molecules that obey octet rulethat obey octet rule

Counting groupsCounting groups

OFOF22 two bonds, two lone pairs two bonds, two lone pairs Total groups = 4Total groups = 4

CFCF44 four bonds, no lone pairs four bonds, no lone pairs Total groups = 4Total groups = 4

Double or triple bonds count as oneDouble or triple bonds count as one

COCO22 has two groups has two groups HCN has two groupsHCN has two groups

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

Summary of possible molecular Summary of possible molecular shapesshapes

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 war the three bonds cancel out – tug of war

stalematestalemate 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, COO is a liquid, CO22

is a gasis 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