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CHEM 200/202
Professor Jing GuOffice: EIS-210
All emails are to be sent to:[email protected]
My office hours will be held on zoom on Tuesday from 9:00 to 11:00 am or by appointment (https://SDSU.zoom.us/s/
99415148959)
UPCOMING IMPORTANT DATES
• Lab Report Due: Calorimetry Part I, Oct.25th-11:59 pm
• OWL Pre-Assignment: Calorimetry Part II, Oct. 25th-11:59 pm
• Pre-Lab: Calorimetry Part II, Oct. 25th-11:59 pm, Oct. 18th-11:59 pm
• SIM: Enthalpy change of a chemical reaction, Oct. 25th-11:59 pm
LECTURE OBJECTIVES• Chapter 7.6
• Use the VSEPR theory to predict molecular structures/shapes.
• Determine the correct VSEPR arrangement from a formula or Lewis structure.
• Recognize the common bond angles in the various VSEPR arrangements.
• Identify variations in the common bond angles in VSPER arrangements due to lone pairs and multiple bonds.
• Use VSEPR structures to determine the polarity of molecules.
LEWIS STRUCTURES & MOLECULAR SHAPE
• Lewis structures show the connectivity (the number and type of bonds) in a molecule.
• Lewis structures do not directly give information on the shape of the molecule.
O••
H H••••O
••H
H
••O••
HH
All are valid Lewis structures for water.
VALENCE SHELL ELECTRON-PAIR REPULSION (VSEPR) THEORY
• An empirical method for determining the molecular shape of a molecule for its Lewis formula.
• Molecular shape can be determined by the number and type of electron groups around each central atom.
• Electron groups:
• Lone pair
• Bonding groups - single, double, triple bonds
ELECTRON REPULSION
• As electrons have the same charge they repel each other.
• Electrons forming a bond will repel other bonding electrons.
• Lone pairs of electrons occupy more space than bonding electron pairs.
• These groupings of electrons “push” each other around to find the proper “space” for each other.
Methane (CH4)
CH HH
HLots of repulsion
CH H
H
HLeast of repulsion (but
not 3D structure)
THE BASE ELECTRON-PAIR https://www.youtube.com/watch?v=Q9-JjyAEqnU&t=28s
TRIGONAL PLANAR ELECTRON-GROUP ARRANGEMENT
Two different forms:AX3: SO3, BF3, NO3-...AX2E: SO2, O3, PbCl2...
trigonal planar bent (v-shaped)
TETRAHEDRAL ELECTRON-GROUP ARRANGEMENT
Three different forms:AX4: CH4, SiCl4, SO42-...AX3E: NH3, PF3, H3O+...AX2E2: H2O, OF2, SCl2...
tetrahedral
bent (v-shaped)
trigonal pyramidal
TRIGONAL BIPYRAMIDAL ELECTRON-GROUP ARRANGEMENT
Four different forms:AX5: PF5, AsF5, SOF4...
AX4E: SF4, XeO2F2, IF4+...AX3E2: ClF3, BrF3...AX2E3: XeF2, I3-,…
trigonal bipyramidalsee-saw
t-shapedlinear
OCTAHEDRAL ELECTRON-GROUP ARRANGEMENT
Three different forms:AX6: SF6, IOF5...
AX5E: BrF5, TeF5-, XeOF4...AX4E2: XeF4, ICl4-...
octahedral
square planar
square pyramidal
# of Groups # of Lone Pairs Molecular Shape
2 0 linear
3 0 trigonal planar
3 1 bent (v-shaped)
4 0 tetrahedral
4 1 trigonal pyramidal
4 2 bent (v-shaped)
5 0 trigonal bipyramidal
5 1 see-saw
5 2 t-shaped
5 3 linear
6 0 octahedral
6 1 square pyramidal
6 2 square planar
linear
trigonal planar
tetrahedral
trigonal bipyramidal
octahedral
STEPS IN DETERMINING A MOLECULAR SHAPE
1. Molecular formula
2. Lewis structure - count all e- groups around the central atom (A)
3. Electron-group arrangement - note lone pairs and double bonds
4. Bond angles - predict the ideal bond angles then account for any deviations due to lone pairs or double bonds
5. Molecular shape (AXmEn) - count bonding & nonbonding e- groups separately
QUESTIONWhich compound or ion is
represented by this molecular view of the gas phase?
NO2-
SCl2CO2SO2
Answer:ABCD
QUESTIONUsing VSPER theory, predict the shape of NI3.
TetrahedralT-shapedSee-saw
Trigonal pyramidalTrigonal planar
Answer:ABCDE
NI••
I
I
trigonal pyramidal
QUESTIONUsing VSPER theory, predict the shape of IF4+.
TetrahedralSquare planar
See-sawOctahedral
Trigonal planar
Answer:ABCDE
I•••• ••••
1+
F
FFF
••
••••
•• ••••
•• ••••
see-saw
SHAPES OF MOLECULES WITH MULTIPLE CENTRAL ATOMS
The rules for the shapes of single atom center molecules
remain effective with multiple central atoms on
larger molecules.
BOND ANGLE DISTORTIONS
• The bond angles expected from the models (ideal) are not always meet in reality. Difference in electron group types will alter the bond angles.
• Relative size of electron groups:
• Lone pairs > multiple bonds > single bonds
• VSPER theory can predict if an actual bond will be smaller or larger than the ideal bond angle.
QUESTIONWhat is the molecular shape of NH3 and the
H-N-H bond angle?
Tetrahedral (109.5°)Trigonal pyramidal (109.5°)
Trigonal planar (120°)Trigonal pyramidal (>109.5°)Trigonal pyramidal (<109.5°)
Answer:ABCDE
https://www.youtube.com/watch?v=xwgid9YuH58https://www.youtube.com/watch?v=8Tl_bDWCAmo
https://www.youtube.com/watch?v=Ip8v87vxSok
MOLECULAR POLARITY• Individual, covalent bonds, are polar when there is a difference in the electronegativity of the bonded atoms (e.g. Si-O, H-F...)
• Molecules can also have polarity, however, this is a product of the aggregate polarity of all the bonds.
• Molecular polarity is influenced by both the bond polarities and the molecular shape.
• The polarity of a molecule is termed the dipole moment (µ), and is measured in units debyes.
• Non-polar molecules do not have dipole moments (µ=0)
MOLECULAR POLARITY
• To determine the molecular polarity draw the proper molecular structure of the compound.
• Indicate the individual bond polarities with polarity arrows.
• Look at arrows as a “tug-of-war”, are they equally balanced or does one side win out over the other.
• Keep in mind that the structure is 3D,
POLAR MOLECULES• Polar molecules are influenced by electric fields; non-polar molecules are not affected by electric fields.
• Polar molecules will self-orient to the applied electric field.
Electric field ON Electric field OFF
MOLECULAR POLARITY• Molecular polarity is the result of
synergistic bond polarities.
• If the polarity of the bonds around the central atom result in an unequal distribution of electronegativity the molecule will be polar.
• The VSEPR structure of the molecule must be considered in determining molecular polarity.
Net polarity:
Net polarity: