Lecture 29 Chapter 9 Sections 4-5

• VSEPR – molecular shape

• Molecular Polarity

• Bond Lengths & Energies

Announcements

• CAPA #16 due tonight• Seminar Tuesday at 11:00

Molecular Shape• The molecular shape describes how the ligands (not the electron

groups) are arranged in space.• This group of molecules all have different shapes.• Electron groups all feel each other, but we can “see” only atoms

Example – H3O+

• What is the steric number?

• Electron group shape?

• How many ligands?

• Molecular shape?

Four

Three

Trigonal pyramid

Tetrahedral

Other Molecular Shapes – p. 366-371

• Steric Number 2: Linear Electron Group Geometry

More Molecular Shapes – p. 366-371

• Steric Number 3: Trigonal Planar Electron Group Geometry

More Molecular Shapes – p. 366-371

• Steric Number 5: Trigonal Bipyramidal Electron Group Geometry

More Molecular Shapes – p. 366-371

• Steric Number 6: Octahedral Electron Group Geometry

Lone pairs are ‘larger’ than bonding pairs

Experiments show that sulfur tetrafluoride has bond angles of 86.8° and 101.5 °.

This is because the lone pair wants to take up more space – it distorts the positions of the other atoms.

Dipole moments

• Recall that polar bonds result from electronegativitydifferences

• Polar bonds can result in polar molecules, depending on the molecule’s geometry

• A polar molecule will align itself in an electric field• Polar things also interact with each other

– “like dissolves like”• The extent to which the molecules align in a field is

referred to as the dipole moment and has the Greek symbol mu, µ.

rqrr=µ

Is H3O+ polar? (Does it have a dipole moment?)

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50%50% 1. Yes

2. No

Hydrogen Halides

Table 9 – 3 Features of Molecular Geometries

Bond Lengths and Energies

• Bond length – the nuclear separation distance where the molecule is most stable.1. The smaller the principle quantum numbers of the valence

orbitals, the shorter the bond.2. The higher the bond multiplicity, the shorter the bond.3. The higher the effective nuclear charge of the bonded atoms, the

shorter the bond.4. The larger the electronegativity difference, the shorter the bond.

• Bond energy – the stability of a chemical bond1. Bond strength increases as more electrons are shared between the

atoms2. Bond strength increases as the electronegativity difference (∆χ)

between bonded atoms increases.3. Bond strength decreases as bonds become longer.

CLOSE BOOKS!Which molecule has the shortest bond length?

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25%25%25%25% 1. F2

2. Cl2

3. Br2

4. I2

Table 9 – 1: Average Bond Lengths

Today• Finish CAPA #16• Read Sections 1-5 of Chapter 10 before Lab• Seminar Tuesday 11:00

Wednesday• Read beginning of Chapter 10• Start CAPA #17

Remember: You are done with the homework when you understand it!