CHEM 200/202

Professor Jing GuOffice: EIS-210

All emails are to be sent to:chem200@sdsu.edu

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

LINEAR ELECTRON-GROUP ARRANGEMENT

Examples: CS2, HCN, BeF2, CO2

linear

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

PRACTICE QUESTION

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.

PRACTICE PROBLEM

IDEAL BOND ANGLES

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:

QUESTIONWhich of the following is a polar molecule?

ANSWER:A - BF3B - XeF2C - CS2D - BrF3

PRACTICE QUESTION