Molecular Shape
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Molecular Shape
description
Molecular Shape. Things to remember. The shape of a molecule is determined by where the nuclei are located. But the nuclei go to certain locations because of the electron pairs. Use the Lewis Structure. Lewis structure is 2-D, but it can help you figure out the 3-D shape. - PowerPoint PPT Presentation
Transcript of Molecular Shape
Molecular Shape
Things to remember
• The shape of a molecule is determined by where the nuclei are located.
• But the nuclei go to certain locations because of the electron pairs.
Use the Lewis Structure
• Lewis structure is 2-D, but it can help you figure out the 3-D shape.
• Learn a few basic shapes and you’ll be ok.
Bonding CapacityBonding CapacityAtomAtom Lewis Lewis
StructureStructure# Unpaired # Unpaired ElectronsElectrons
Bonding Bonding CapacityCapacity
H H HH 11 11
F, Cl, Br, IF, Cl, Br, I ·F:·F: 77 11
C, SiC, Si ·C··C· 44 44
N, PN, P ·N··N· 55 33
O, SO, S ·O:·O: 66 22
Ne, Ar, KrNe, Ar, Kr :Ne::Ne: 88 00
..
....
....
..
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··········
Basic Shapes
• Linear: all diatomics, CO, H2, N2, O2, etc. and also some triatomics: CO2
• Bent: some triatomics: H2O, H2S, H2Se, H2Te, OF2
• Pyramidal: NH3, PH3, NI3
• Tetrahedral: CH4, CCl4, CBr4, CH3F, etc.
VSEPR Model• Draw the Lewis structure.• Count the regions of electron density on the
central atom. (How many sides have electron density?)• Each single, double, &/or triple bond counts
as 1 region of high electron density.• Each nonbonding electron pair counts as 1
region of high electron density.• Count the number of atoms bonded to the
central atom.
Orientation of Regions of ED
Total Number of Regions of ED
Orientation
1 or 2 Linear: 2 electron pairs can be 180 apart
3 Planar Triangular: 3 electron pairs - 120 apart
4 Tetrahedral: 3-D, 4 electron pairs - 109 apart
Summary of Molecular Shapes
# of regions of electron density
# of bonded atoms
shape
2 2 Linear
3 3 Trigonal planar*
4 4 Tetrahedral
4 3 Trigonal pyramid
4 2 Bent
2-Atom Molecules
• Atoms are right next to each other.
• 2 points make a line – these are linear!
3-Atom Molecules
• Most likely Most likely possibilities are possibilities are linear and bent.linear and bent.
Linear Linear Bent Bent
3-Atom Molecules
• Triangular?
Well, isn’t that another way of Well, isn’t that another way of saying bent?saying bent?
4-Atom Molecules
• Two possibilities:–Trigonal Planar – in 1 plane
–Trigonal Pyramidal
4-Atom Molecules: Trigonal Planar
Bond angles = Bond angles = 120120..
All 4 atoms lie in All 4 atoms lie in the same planethe same plane
You aren’t likely to see this. These You aren’t likely to see this. These molecules don’t obey the octet rule!molecules don’t obey the octet rule!
4-Atom Molecules: AX3
The shape you are The shape you are most likely to run most likely to run into.into.
5-Atom Molecules: AX4
5 Atoms & Tetrahedrons
Tetrahedral Tetrahedral means 4 faces.means 4 faces.
1 atom is at the 1 atom is at the center & 4 are center & 4 are at the points.at the points.
Molecular Shape
• Determined by overlap of orbitals.
• Shape is determined by two factors:
–The total number of atoms The total number of atoms andand
–The number of electron pairs The number of electron pairs in in different locationsdifferent locations on the central on the central atom.atom.
–Classify electron pairs as bonding or Classify electron pairs as bonding or nonbonding.nonbonding.
Molecular Shape & VSEPR
• Electron pairs repel each other. They want to be as far apart from each other as they can.
• Nonbonding pairs take up a little more room than bonding pairs.
Orientation of Electron Pairs
Total Number of Electron Pairs
Orientation
1 or 2 Linear: 2 electron pairs can be 180 apart
3 Planar Triangular: 3 electron pairs - 120 apart
4 Tetrahedral: 3-D, 4 electron pairs - 109 apart
CO2
Lewis structure =Lewis structure = O :: C :: O O :: C :: O....
.... ....
....
4 bonding pairs.4 bonding pairs.But only But only 2 regions2 regions of electron of electron density density 180180 apart. CO apart. CO22 is is
linear.linear.
BF3: Trigonal Planar
• B has 3 valence electrons. It’s a very small atom. Each F has 7 valence electrons. Total = 3 X 7 + 3 = 24.
: F : B : F : B ....
........ FF
FF
....
....
....
....
.... ........
These molecules These molecules don’t obey the octet don’t obey the octet rule, so you aren’t rule, so you aren’t likely to see them.likely to see them.3 regions3 regionsFBF = 120FBF = 120
CH4
Lewis structure =Lewis structure = H : C : H H : C : HHH
HH........
4 bonding pairs4 bonding pairs4 regions of electron density4 regions of electron densityElectron pairs are 109Electron pairs are 109 apart. apart.
CH4 = a 5-atom molecule
NH3
Lewis Structure =Lewis Structure = H : N : H H : N : H
HH
....
....
3 bonding pairs3 bonding pairs1 nonbonding pair1 nonbonding pair4 regions. 1094 regions. 109 apart.apart.
4-atom 4-atom molecule.molecule.
Shape = Shape = trigonal pyramidtrigonal pyramid
HNH = a bit HNH = a bit less than 109less than 109
Looks like a pyramid with N at the Looks like a pyramid with N at the top & a triangular base.top & a triangular base.
Lewis Structure of HLewis Structure of H22O =O = H:O:H H:O:H....
....H2O
2 bonding pairs between the O and 2 bonding pairs between the O and the H’sthe H’s2 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, but the nonbonding pairs spread out a bit more and squeeze the bonding pairs together.
Summary of Molecular Shapes
• Start with Lewis Structure!
• Look at number of regions of electron density on central atom.
• Look at number of atoms bonded to central atom.
Summary of Molecular Shapes
# of regions of electron density
# of bonded atoms
shape
2 2 Linear
3 3 Trigonal planar*
4 4 Tetrahedral
4 3 Trigonal pyramid
4 2 Bent
Molecular Polarity
• Look at the type of bonds in the molecule.
• Look at the shape of the molecule.
• A polar molecular must contain A polar molecular must contain polar bonds & it must be polar bonds & it must be asymmetric (NOT symmetric).asymmetric (NOT symmetric).
Molecular Polarity• If molecule is symmetric, the electrical
charge at any point on 1 side = electrical charge at matching point on opposite side.
• the “pull” of one polar bond is offset by the “pull” of another polar bond.– It’s a tug-of-war that no one can win!
Symmetric Symmetric Molecules Molecules NonpolarNonpolar
NonpolarNonpolar
AsymmetricAsymmetricMoleculesMolecules PolarPolar
