Topic: Molecular Shape

Do Now: Draw Lewis Dot Diagram for the following two compounds

1. CH2F2

2. CaO[ Ca ]+2

Use the Lewis Structure• Lewis structure is 2-D, but can

help figure out 3-D shape

H O H

REMINDER:Bonding Capacity

00:Ne:Ne, Ar, Kr

22·O:O, S

33·N·N, P

44·C·C, Si

11·F:F, Cl, Br, I

11HH

Bonding Capacity

# Unpaired Electrons

Lewis Structure

Atom

.

..

..

.

......·····

Molecular Shape • Determined by overlap of orbitals

• Shape determined by two factors:

1. # e- groups (pairs) around the central atom – pay close attention to the “free electron pairs”

2. total # atoms bonded to central atom

Free electron pairs

Central Atom

VSEPR(A model used to predict the shape of individual molecules based on the extent of electron-pair

electrostatic repulsion)

• VSEPR= Valence shell electron pair repulsion: –Basically free electrons will repel one

another causing the molecule to bend

2 atoms (no central atom)

• Always linear

– Examples• H2

• O2

• N2

• HCl

3 atoms• Can be linear or bent

–Must look at central atom • If free electrons, then will be

bent

Central atomNo free electrons so linear

Central atom Free electrons so bent

4 atoms that form triangles• Look at central atom

–No free electrons = trigonal planar

- Free electrons = trigonal pyramidal

4-Atom Molecules:

Planar = no real central atom, no free electrons either

5 atom molecules

• Always tetrahedral

– Examples• CCl4

• CH4

• CH2I2

Summary of Molecular Shapes• Start with Lewis Structure!

• Look at total number of atoms• Look at # of atoms bonded to

central atom• Look at free pair(s) of electrons

around central atom

Symmetrical Molecules Nonpolar

Nonpolar

Asymmetric Molecules Polar Bent & trigonal pyramidal are always polar

Let’s try a few… Name the SHAPE and

POLARITY

BENTPOLAR

Trigonal PlanarPOLAR

TetrahedralPOLAR

Trigonal PyramidalPOLAR

linearPOLAR

linearPOLAR

bentPOLAR

tetrahedralNONPOLAR