1

Recap VSEPR Theory1. Draw Lewis Structure.2. Count number of electron pairs.3. Determine the arrangement of electron

pairs.4. Use atom positions to name molecular

geometry.

2

Three areas of electrons: Trigonal Planar• Remove one arm from the electron pair

arrangement for each lone pair present.

Recap

Figure 10.4 Silberberg

SO3 SO2

3

Four areas of electrons: Tetrahedral

Recap

Figure 10.5 Silberberg

CH4 NH3 H2O

4

Electron Pair Arrangements

• Five electron pairs:– Atoms at the corners of a trigonal

bipyramid.– Some electron pairs separated by 120

degrees, other by 90 degrees.– Called trigonal bipyramidal.

Figure 10.2 Silberberg

5

Electron Pair Arrangements

• Six electron pairs:– Atoms at the corners of an octahedron.– 90 degrees between electron pairs.– Called octahedral.

Figure 10.2 Silberberg

6

Where do lone pairs go?• Five electron pairs:

– If one lone pair present is it in an axial or equatorial position?

– Repulsion: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

Equitorial

Axial

Structures derived from a trigonal bipyramid

Molecular Geometry

Figure 10.7 Silberberg

8

Structures derived from an octahedron

Molecular Geometry

Figure 10.8 Silberberg

All positions are identical

  Number of lone electron pairs0 1 2 3

3

trigonal planar bent    4

tetrahedraltrigonal

pyramidal bent  5

trigonal bipyramidal see-saw T-shaped linear

6

octahedralsquare

pyramidal square planar T-shaped

Tota

l num

ber o

f ele

ctro

n pa

irsSummary of Molecular Geometry

10

Molecular Geometry and Polarity

BF3 ClF3F Cl

F

F

PolarNon-Polar

F B

F

F

11

Molecular Geometry and Polarity

SiF4 SF4

PolarNon-Polar

F

SiF F

F

F

SF F

F

• By the end of this lecture, you should:− be able predict the molecular geometry of

molecules and ions containing a central atom

− be able to name the geometry of a given species

− understand the positioning of lone pairs to minimise repulsion

− be able to predict the presence of a permanent dipole moment

− be able to complete the worksheet (if you haven’t already done so…)

12

Learning Outcomes:

13

Questions to complete for next lecture:

1. Give the geometry of the following polyatomic ions and give approximate bond angles.a) AsF6

-

b) PBr4-

c) SF5-

d) SO42-

e) ICl2-

f) N3-

2. A binary compound contains two different elements. What molecular geometries would result in binary compounds that are polar?

3. What molecular geometries would result in binary compounds that are non-polar?