Molecular Shape

Things to remember

shape of molecule:– determined by location of nuclei – nuclei go to certain locations because

of its electron pairs

Use the Lewis Structure

• Lewis structure is 2-D, but can help figure out 3-D shape

Bonding CapacityBonding Capacity

0000:Ne::Ne:Ne, Ar, KrNe, Ar, Kr

2222·O:·O:O, SO, S

3333·N··N·N, PN, P

4444·C··C·C, SiC, Si

1111·F:·F:F, Cl, Br, IF, Cl, Br, I

1111HHH H

Bonding Bonding CapacityCapacity

# Unpaired # Unpaired ElectronsElectrons

Lewis Lewis StructureStructure

AtomAtom

.

..

..

.

......···

··

Molecular Shape

• Determined by overlap of orbitals

• Shape determined by two factors:

1.1. total # atoms total # atoms & &

2.2. # e# e-- pairs pairs in different locationsin different locations on on central atomcentral atom

Classify electron pairs as bonding or Classify electron pairs as bonding or nonbondingnonbonding

Molecular Shape & VSEPR

• Electron pairs repel each other: –want to be as far apart from each other as can be

• Nonbonding pairs take up a little more room than bonding pairs

Orientation of Electron Pairs

Tetrahedral: 3-D, 4 electron pairs - 109 apart

4

Planar Triangular: 3 electron pairs - 120 apart

3

Linear: 2 electron pairs can be 180 apart

1 or 2

OrientationTotal Number of Electron Pairs

2-Atom Molecules

• Atoms located right next to each other

• linear molecules!

3-Atom Molecules

• linear or bentlinear or bent

LinearLinear

Bent Bent

CO2

Lewis structure:Lewis structure: O :: C :: O O :: C :: O....

.... ....

....

4 bonding pairs4 bonding pairsBut onlyBut only 2 regions2 regions of electron density: of electron density: 180180 apart so CO apart so CO22 is linear is linear

Lewis Structure of HLewis Structure of H22O =O = H:O:H H:O:H....

....H2O

•2 bonding pairs between O and the H’s2 bonding pairs between O and the H’s•2 nonbonding pairs on the O2 nonbonding pairs on the O•4 different regions of electron density4 different regions of electron density

Count up the electron pairs:Count up the electron pairs:

H2O

• 4 electron pairs are 109 apart

• nonbonding pairs spread out a bit more

– squeeze the bonding pairs together

– bent shape

H – O H

http://www.chemistry210.com/notes/u01s07b.htm

3-Atom Molecules

• Triangular?

Yes, ozone (OYes, ozone (O33) is triangular ) is triangular

4-Atom Molecules

• Two possibilities:–Trigonal Planar – in 1 plane

–Trigonal Pyramidal

4-Atom Molecules: Trigonal Planar

Bond angles Bond angles =120=120

All 4 atoms lie in All 4 atoms lie in the same planethe same plane

You aren’t likely to see this because theseYou aren’t likely to see this because these molecules don’t obey the octet rule!molecules don’t obey the octet rule!

BF3: Trigonal Planar• B has 3 valence electrons

– very small atom e– each F has 7 valence electrons – total = 3 X 7 + 3 = 24 e-

: F : B : F : B ....

........FF

FF....

....

....

....

.... ........

These molecules don’t These molecules don’t obey octet rule - so obey octet rule - so aren’t likely to see aren’t likely to see themthem - 3 regions- 3 regions - FBF = 120- FBF = 120

4-Atom Molecules: AX3

shape most likely to seeshape most likely to see

NH3

Lewis Structure:Lewis Structure: H : N : H H : N : H

HH

....

....

•3 bonding pairs3 bonding pairs•1 nonbonding pair1 nonbonding pair•4 regions 4 regions •109109 apart apart

4-atom molecule4-atom molecule

•Shape = trigonal Shape = trigonal pyramidpyramid• A bit less than A bit less than 109109

5-Atom Molecules: AX4

5 Atoms & Tetrahedrons

Tetrahedral means 4 Tetrahedral means 4 faces:faces:- 1 atom in center - 1 atom in center - 4 atoms at the points- 4 atoms at the points

CH4 = a 5-atom molecule

CH4

Lewis structure =Lewis structure = H : C : H H : C : HHH

HH........

•4 bonding pairs4 bonding pairs•4 regions of electron density4 regions of electron density•electron pairs are 109electron pairs are 109 apart apart

CH4 = a 5-atom molecule

Summary of Molecular Shapes

• Start with Lewis Structure!

• Look at # regions of electron density on central atom

• Look at # atoms bonded to central atom

Molecular Polarity

• Look at type bonds in molecule

• Look at shape of molecule

• nonpolar molecular must be nonpolar molecular must be symmetricalsymmetrical

Molecular Polarity• molecule is symmetric if:

– electrical charge on 1 side = electrical charge at matching point on opposite side

–“pull” of one polar bond is offset by “pull” of another polar bond

– tug-of-war that can't be won!

Symmetrical Symmetrical Molecules Molecules NonpolarNonpolar

NonpolarNonpolar

AsymmetricAsymmetricMoleculesMolecules PolarPolar