1. Molecular Geometry and VSEPR Theory
Foothill Chemistry

2. Molecular Geometry
The properties of molecules depend not only on the bonding of atoms but also on molecular geometry: the three-dimensional arrangement of a molecule’s atoms.
The polarity of each bond, along with the geometry of the molecule, determines molecular polarity, or the uneven distribution of molecular shape.
Molecular polarity strongly influences the forces that act between molecules in liquids and solids.

3. VSEPR V – Valence S – Shell E – Electron P – Pair R – Repulsion
It is based on this theory that we understand the three dimensional structure of molecules.
VSEPR theory states that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible.

4. Diatomic Molecules
As shown at right, diatomic molecules, like those of (a) hydrogen, H2, and (b) hydrogen chloride, HCl, can only be linear because they consist of only two atoms.
To predict the geometries of more-complicated molecules, one must consider the locations of all electron pairs surrounding the bonding atoms. This type of prediction is the basis of VSEPR theory.

5. VSEPR Geometry Linear Geometry example: BeF2
The central beryllium atom is surrounded by only the two electron pairs it shares with the fluorine atoms.
According to VSEPR, the shared pairs will be as far away from each other as possible, so the bonds to fluorine will be 180° apart from each other.
The molecule will therefore be linear:
Representing the central atom in a molecule by A and the atoms bonded to the central atom by B, then according to VSEPR theory, BeF2 is an example of an AB2 molecule, which is linear.

6. VSEPR GEOMETRY – AB Models (Central atom has no unbound electrons)
Shape
Description
AB2
Linear
the two outer atoms are 180° from each other
AB3
Trigonal Planar
– three outer atoms at the corners of an equilateral triangle
– each outer atom is 120° from the other two outer atoms
AB4
Tetrahedral
– maximum distance between electrons requires 3D structure with 109.5° between each outer atom
– each outer atom is 109.5° from the other outer atoms
AB5
Trigonal Bipyramidal
– equatorial positions: corners of planar triangle – 3 of outer atoms are at equatorial positions, 120° from each other
– axial positions: above and below central atom – 2 atoms are at axial positions, 90° from equatorial atoms
AB6
Octahedral
all outer atoms are 90° away from each other

7. CO2 – Carbon Dioxide (AB2)
Common VSEPR Geometry
CH4 - Methane (AB4)
Tetrahedral
CO2 – Carbon Dioxide (AB2)
Linear

8. VSEPR Geometry (Central atom has no unbound electrons)

9. VSEPR Geometry (Central atom has no unbound electrons)

10. VSEPR ABE Models (Central atom has unbound electrons)
Geometry
Shape
Description
AB2E
Bent
– start with AB3 molecule (trigonal planar) and replace a B atom w/ lone pair
– lone pair electrons push bonding electrons away → bond angles are now less than 120°
AB3E
Trigonal Pyramidal
– start with AB4 molecule (tetrahedral) and replace a B atom w/ lone pair
– lone pair electrons push bonding electrons away → bond angles are now less than 109.5°
AB2E2
– start with AB4 molecule (tetrahedral) and replace 2 B atoms with 2 lone pairs
– lone pair electrons repel each other and the bonding electrons → bond angles are now less than 109.5°
AB5E
Square Pyramidal
– start with AB6 (octahedral) and replace one B atom with one lone pair
AB4E2
Square Planar
- Start with AB5E, and replace the B atom that is opposite the E atom with one lone pair

11. Common VSEPR Geometry H2O Water (AB2E2) NH3 Ammonia (AB3E) Bent
Trigonal Pyrimidal

12. VSEPR Geometry (Central atom has unbound electrons)

13. VSEPR Geometry (Central atom has unbound electrons)

14. VSEPR Geometry (Central atom has unbound electrons)