Molecular Shape

Things to remember

shape of molecule influenced 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 eclassify e-- pairs as bonding or non-bonding pairs as bonding or non-bonding

Molecular Shape & VSEPR

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

• non-bonding pairs take up a little more room than bonding pairs

Orientation of Electron Pairs

tetrahedral: 3-D, 4 e- pairs 109 apart

4

planar Triangular: 3 electron pairs 120 apart

3

linear: 2 electron pairs 180 apart

1 or 2

OrientationTotal # of electron pairs

2-Atom Molecules

• atoms located 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:O: HH

....

....H2O

•2 bonding pairs between each O and H2 bonding pairs between each O and H•2 nonbonding pairs on O2 nonbonding pairs on 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 bonding pairs together so

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

arenaren’’t likely to see this because theset likely to see this because these molecules molecules dondon’’t obey octet rule!t obey 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 These molecules dondon’’t obey octet rule t obey octet rule - so aren- so aren’’t likely to t likely to see themsee them - 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 non-bonding pair1 non-bonding pair•4 regions 4 regions •little less than 109little less than 109 apart apart•shape: trigonal pyramidalshape: trigonal pyramidal

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 e- density on central atom

• look at # atoms bonded to central atom

Molecular Polarity

• Look at shape of molecule:–non-polar molecular is symmetricalnon-polar molecular is symmetrical

–polar molecular is asymmetricalpolar molecular is asymmetrical

Molecular Polarity• molecule is symmetric if:

– electrical charge on 1 side is same as electrical charge at matching point on opposite side

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

Symmetrical Symmetrical Molecules are Molecules are Non-polarNon-polar

NonpolarNonpolar

AsymmetricAsymmetricMolecules areMolecules arePolarPolar