Algorithm for the prediction of molecular Geometry:

NH3 H N H

H

°°Lewis Structure

Electron pair Geometry

Molecular Geometry

Tetrahedral Trigonal Pyramid

To describe the shape of a molecule, one always indicates the shape of the molecule, instead of the electron pair geometry.

Electron pair geometry on the basis of the electron pair number

Number of Geometry of Bonds Bond e- pairs e- pairs enlace Geometry Angle

2

3

4

5

6

Linear 180º

Trigonal 120ºplanar

Tetrahedral 109.5º

trigonal 120ºBipyramidal 90º

Octahedral 90º 180º

• The molecular geometry is described in terms of the angular array of the bonding pairs.

• For multiple bonded molecules, the VSEPR model considers these as one single bonding pair to predict the geometry.

• The following table sums up some of the geometries observed for ABn type molecules.

Electron Pairs Geometry and Molecular Shapes

Number of Arrangement Non bonding e- pairs e- pairs of bonding e- pairs e- pairs Geometry example

2 pairs

3 pairs

4 pairs

2 0

3 0

2 1

4 0

3 1

2 2

Tetrahedral

Trigonalplanar

Lineal linear

Trigonal planar

Angular

Tetrahedral

Trigonal pyramidal

Angular

Example: Predict the molecular shape of silicium tetrachloride, SiCl4

Si

Cl

Cl

ClCl

1.-Determinate the Lewis structuree of the compound or ion and deduce the valence electron pairs of the central atom, whether they are bound or not. A double bond is considered equal to a single one

2.-Distribute the electron pairs in space so that the repulsions are minimized. (Lookup the electronic pairs geometry table) . When the non bonding electron pairs can be situated in more than one non equivalent position, choose the one that minimizes repulsions.

The molecular geometry is determined by the position of the peripheral atoms bound to the central one.

For the SiCl4 molecule, a tetrahedral structure is predicted, with bond angles of aproximately 109.5°

Example: Predict the molecular shape of Phosphorous trichloride, PCl3

1.-Write up the Lewis stucture of the molecule:

°°The central atom carries four pairs of valence electrons (one Lone Pair and three bound ones), the ideal distribution in this case is a tetrahedron:

(See table: Electron pairs Geometry)

.

In this specific case, trhe tetrahedron is distorted due to the ps-pe effect which does that the Cl-P-Cl angle be smaller than 109.5° and that the geometry is the one of a trigonal pyramid.