Upload
robert-marshall
View
23
Download
0
Tags:
Embed Size (px)
DESCRIPTION
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. Atom. Lewis Structure. # Unpaired Electrons. - PowerPoint PPT Presentation
Citation preview
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
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