Octet Exceptions & VSEPR Model March 5, 2007

More Octet Exceptions Some molecules have too many electrons to hold to the octet rules. SF6 is an example Only elements in period 3 and higher can exceed the octet rule. If there is a question as to where the extra electrons go, put them on the central atom. XeCl2

VSEPR Model Predicting the 3-D molecular structure of an atom. The model attempts to minimize electron-pair repulsions. Begin with the Lewis Structure! Predict the arrangement of the electron pairs to minimize repulsions. Determine the positions of the bonded atoms. Determine the molecular structure name.

4 Electron Groups Tetrahedral arrangement NOT 90º 109.5º angle

4 Electron Groups (1 lone pair) NH3 (ammonia)

4 Electron Groups (2 Lone Pairs) H2O (water)

Lone Pairs and Bond Angles The more lone pairs, the smaller the bond angle.

Exam Practice Which of the following molecules or ions has the smallest bond angle? or the largest? a. H2O b. HCN c. O3 d. CH4 e. NO3 smallest largest

5 Electron Groups PCl5 – trigonal bipyramid bond angles of 90, 120 (and 180) http://www.elmhurst.edu/~chm/vchembook/220trigbipy.html

6 Electron Groups