Organic chemistry

1. GEOMETRY OF
MOLECULES
GENERAL CHEMISTRY I

2. INTRODUCTION
•Although we cannot see the molecule of a
substance with our naked eyes, these minute
particles do have a particular shape or
geometry.The shape of molecules greatly
influences physical and chemical properties,
including melting point, boiling point, and
reactivity of the substances.

3. What is VSEPRTheory?
•Valence Shell Electron Pair Repulsion (VSEPR)
theory was developed in order to predict the
geometry of covalent compounds.The principle
ofVSEPR states that valence electrons repel one
another (they are negatively charged so they
repel each other).The geometry around the
central atom will be such as to minimize the
repulsion - minimizing repulsion means
minimizing stress.

4. Molecular geometry is the arrangement of
atoms of a molecule in space.
•VSPER model includes: A= central atom, X=
bonding atom and E= lone pair of electrons.
Steric number (SN) is used to predict
geometries.
•Steric number (SN) = (# of atoms bonded to
the central atom) + (# of lone pair electrons)
(you count one pair as one)

5. Note:
•When considering electron- pair repulsion, double and
triple bonds can be treated as single bonds.
• Number of lone pairs and atoms bonded to the central atom is important,
not the BONDS to central atom.

6. The bond length and bond angle describe the size and shape of the molecule

8. You can predict the molecular geometry of
molecules using the following steps.
•Step 1. Draw the appropriate Lewis structure.
• Step 2. Determine the number of electron
groups around the central atom and identify
each as a bonding pair or a lone pair.
•Step 3. Determine the molecular geometry

9. Molecules with lone pairs
Electrons in bonds have less “spatial
distribution” than lone pairs, meaning;
•Electrons in bonds take up less space,
•Lone-pair electrons take up more space,
and therefore experience more repulsion.