Week 5

This week in AP Chemistry we learned many of the characteristics of the VSEPR (Valence Shell Electron Pair Repulsion) Theory. This theory helps to explain the reasons why molecules take on the shapes that they do, especially simple central-surrounding atom molecules with covalent bonds such as CH4 and H2O. This theory states that the shapes of these molecules depend strongly on the interactions of the valence electrons of the central atom. It classifies the electron pairs into two categories: bonded and lone pairs. The properties of these electron pairs determine the three scientific classifications for molecules of this type: molecular class, electron domain geometry and molecular domain geometry.

Molecules are described in various ways, including the following. Molecular classes, such as AB2E3 are very concise summaries of the shape of a molecule and can be used to determine the electron domain geometry and molecular domain geometry for that particular molecule. In this notation an A is used to represent the central atom, the B is used to represent a bonded electron pair and the subscript determines the number of these pairs, and the E is used to represent a lone pair of electrons and its subscript shows the number of these pairs. For this notation, the subscripts of B and E should always sum to the number of electron pairs of the central atom. Electron domain geometry is the overall shape of the central atom's electrons, making no significant distinction between bonded and lone pairs. Molecules with central atoms with 2,3,4,5 and 6 electron pairs are classified as linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral, respectively. The molecular domain geometry of a molecule is a sub set of its electron domain geometry and are determined by the number of lone pairs within the original structure. A site I used to review these different shapes is listed below.

Two models we used in class in order to show
the interactions of the valence electrons

Throughout middle school and into high school, I have always seen H2O in its bent molecular domain geometry while CO2 was in a linear form, and I was puzzled why this happened. I was very curious to see whether there would be a simple set of rules such as I have found in VESPR or a complicated set of strange rule-breakers requiring large amounts of memorization. I am glad that there is a simplistic, although occasionally difficult, way to determine the shape that a molecule will form, knowing nothing aside from what the molecule consists of.

A significant point was made this week that in VSEPR models the lone pairs of the central atom are significantly large than bonded pairs, as a result of the forces on them. This means that the lone pairs of the molecule will repel other pairs farther than the bonded pairs. In turn, the molecule retains its overall molecular domain geometry, while the angles involved in these shapes become much more complicated, straying from clean numbers such as 90, 120, 180 and 109.5. Instead, without further calculations, you may only estimate that it will be slightly more or less than these original numbers. I hope that sometime in the future we will be able to calculate these angles more precisely.

A good summary of VSEPR characteristics:
http://misterguch.brinkster.net/VSEPR.html