Where are the electrons in a water molecule? Do they move equally around all three atoms or do they spend more
time near just one atom, or maybe two?
Questions like these can be answered by appealing
to quantum mechanics. This theory says that
electron positions can be calculating a molecule's wave function, and then converting this into another function called
electron density. The numerical behavior of the electron
density function tells us where the electrons are located.
Luckily, you will not need to learn how to do quantum mechanical calculations. Standardized computer programs can carry out the calculations for you. They can also convert the numerical values into graphs like the following:
Your goal is to learn how chemists use electron density
data. The first five essays describe things that every student should
know: what chemists mean by "electron density," important
graphing techniques, universal features of electron density clouds,
and two important applications of electron density in chemistry.
The final essay tackles more difficult material and can be skipped (it shows some additional ways that chemists use electron density data, but we will not use these techniques in Chem 201/202).