Day 19

Context

Scientists are constantly trying to model things that they can't see and it can result in a progression of theories due to new experiments. We have learned that the first real model of the atom was the raisin pudding (or plumb pudding) model which was replaced by the solar system model. The solar system model turned out to not be consistent with experiment either and so the quantum mechanical model was proposed. The quantum mechanical model is the current model and retains some of the ideas and terminology of the original Bohr model, but it is probabilistic in nature and uses quantum mechanics to describe the atom.

Explanation

Around 1925 two young scientists, Heisenberg and Schrodinger, independently proposed mathematical models to describe electrons in the atom. The models were equivalent and now are referred to as the original formulations of quantum mechanics. We will concentrate on the part of the theory that describes where the electron is likely to be in space around the nucleus. The theory completely rejects the idea that electrons orbit the nucleus and only gives regions of space where the electron is most likely to be.

After this class you should be able to:

• Name the currently accepted atomic theory.
• State the difference between a continuous spectrum and a line spectrum.
• Use the position of an electron to illustrate the probabilistic nature of quantum mechanics.

Probability of finding an electron

Quantum mechanics is a model that uses probability. It allows us to calculate the probability of finding an electron in a specified region of space for a given energy state, but does not allow us to know exactly where the electron is located. The region of space where the electron is likely to be found changes when the electron changes energy state.

One way of understanding this is to say that the mathematical equations can be used to calculate the most likely spot for an electron to be at a given instant. The time can be changed and the position calculated again. If a dot is placed around the nucleus for each of these calcualations, and it is done many times, we will get a picture of dots around the nucleus. Where there are more dots it will be more likely to find the electron. This process will then allow a calculation of the region in space where the electron is most likely to be. The equations are different for each energy state and so the whole thing must be done again for each energy state. These regions where there are a lot of dots are regions where the probability is high of finding an electron. We don't know exactly where an electron is, so there is uncertainty, but we can predict where it is likely to be or where the probability of finding it is high. See the box

The box on the right illustrates this process.

Richard Feynman said that it isn't a matter of liking a theory, or if the theory makes common sense, but whether or not it agrees with experiments that determines if it is a good theory. Quantum mechanics is kind of complicated and cannot be associated with common sense, but it works!

The good news is that a few simplifications of the complicated mathematical theory results in a useable model that allows us to explain much of the chemistry that we observe. If we just remember that there is a large positive nucleus with much smaller electrons outside of that nucleus, as predicted by the quantum mechanical model, it will go a long way. One of the results of that is that any energy that the atom gets will affect the electrons a lot more than the nucleus. If anything moves in the atom it will be the electrons. This is the basic principle that governs chemistry.

Homework

There is no homework associated with Day 19.