Molecular Geometry: The VSEPR Model

 (read section 7.1 & 7.2)

Assumption:

VSEPR (Valence Shell Electron Pair Repulsion) Rules:

1. Electron pairs tend to ____________________. For a given no. of electron pairs

around the central atom certain geometries will minimize electron pair

repulsions.

No. of e- pairs	Ideal Geometry of e- prs.	Examples
2	linear	CO2, BeCl2
3	trigonal planar	BF3 (CN = 3, trig. planar) SO2 (CN = 2, bent)
4	tetrahedral	CH4 (CN = 4, tetrahed.) NH3 (CN = 3, trig. pyr.) H2O (CN = 2, bent)
5	trig. bipyramidal	PF5
6	octahedral	SF6

CN = coordination number = number of atoms bonded to the central atom

No. of e^- pairs is the no. of lone pairs + bonding pairs counting multiple bonds as only one pair. For example, CO₂ has two double bonds so we say that it has "two pairs of electrons" on the central atom and therefore linear geometry.

(see board)

2. Repulsions increase in the order _________________________.

Lone pairs are "bigger" than bonding pairs since they are attracted to only one

nucleus rather than two. So the electron cloud spreads out more.

a. When lone pairs are present, the bond angles are ___________ than predicted by

rule 1.

BF₃ (120^o) vs. SO₂ (119^o, exp.; 114^o, Hyperchem (ab initio))

CH₄ (109.5^o) vs NH₃ (107^o, exp.; 109^o, Hyperchem (AM1))

b. Lone pairs chose the ________________, e.g. equatorial in trigonal bipyramidal.

PCl₅ has trigonal bipyramidal geometry:

But now consider BrF₃

BrF₃ has a distorted T-shape. The bond angle is 81^o (Hyperchem, AM1); 86^o (exp.). This is smaller than the expected angle of 90^o and is due to repulsion by the lone pairs.

c. If all sites are equal, lone pairs will be 180^o to each other.

SF₆

 But consider XeF₄

Other examples:

H₂O

SO₃

SO₃^2-

SF₄