Section 8 3 Bonding Theories VSEPR Theory Electron

Section 8. 3 Bonding Theories

VSEPR Theory Electron dot structures fail to reflect the three dimensional shapes of the molecules. Valence Shell Electron Pair Repulsion Theory (VSEPR) According to VSEPR theory, the repulsion between electron pairs causes molecular shapes to adjust so that the valence electron pairs stay as far apart as possible.

Unshared pairs of electrons – no bonding atom is vying for these unshared electrons so they are held closer to the central atom than are the bonding pairs. The unshared pair strongly repels the bonding pairs pushing them together. In predicting molecular shapes, it may be useful to start with an electron dot structure. The electron dot structure shows both the bonding and nonbonding pairs of electrons around the central atom.

VSEPR Theory When using VSEPR theory to predict molecular shape, double and triple bonds are viewed as single bonds. When 4 pairs of electrons must be accommodated around the central atom – tetrahedral (109. 5º) When 3 pairs – a trigonal planar maximizes space (120º) When 2 pairs – a linear arrangement (180º)

VSEPR for methane (a gas): H H C H H This 2 -dimensional drawing does not show a true representation of the chemical arrangement. • Single bonds fill atoms. • There are 4 pairs of electrons pushing away. • The furthest they can get away is 109. 5°

4 atoms bonded • Basic shape is tetrahedral. • A pyramid with a triangular base. • Same shape for everything with 4 pairs. 109. 5° VSEPR Structure

3 bonded - 1 lone pair • Still basic tetrahedral but you can’t see the electron pair. • Shape is called trigonal pyramidal. H N H H 107° VSEPR Structure

3 atoms no lone pair • The farthest you can the H electron pair apart is 120º. • Shape is flat and called H trigonal planar. • Will require 1 double bond 120° C O

2 bonded - 2 lone pair • Still basic tetrahedral but you can’t see the 2 lone pair. • Shape is called bent. H O H 105° VSEPR Structure

2 atoms no lone pair • With three atoms the farthest they can get apart is 180°. • Shape called linear. • Will require 2 double bonds or one triple bond 180° O C O

pages 232 - 233 • • • Ammonia (NH 3) = 107 o Water (H 2 O) = 105 o Carbon dioxide (CO 2) = 180 o Methane (CH 4) = 109. 5 o Boron fluoride (BF 3) = 120 o