0.5 Electron orbitals and electron configurations in atoms  (Page 3/10)

The concept of wave-like motion for electrons has been very difficult to imagine or visualize. What does it mean for a particle to move like a wave? This is very subtle, and we will discuss it later in this study. But we can visualize it. Very recently, scanning tunneling microscopy (STM) has been used to take images that clearly reveal this wave-like character. The STM mechanism can be used to literally pick up and place metal atoms in specific arrangements on metal surfaces. For example, iron atoms have been arranged to form a closed circle on a copper surface. An image of the resultant structure then taken using the STM shows not only the ring of iron atoms but also conspicuous waves inside the ring, which result from the motion of electrons moving within the ring and reflecting off of the walls formed by the iron atoms. Two of the original images taken at IBM of these so-called “quantum corrals” are shown in [link] .

Interpretation of the wave motion of electrons is a very complicated proposition, and we will only deal at present with a single important consequence, namely the uncertainty principle . A property of wave motion is that, unlike a particle, the wave does not have a definite position at a single point in space. By contrast, our everyday experience with particles is that the location of a particle is precise. We can look at something and determine where it is with a great deal of certainty. But our experiments tell us that the electron travels as a wave, and we cannot determine precisely the location of a wave. We must conclude that we cannot determine the precise location of an electron in an atom. This is, for our purposes, the “uncertainty principle” arising from the branch of Physics called quantum mechanics. Even though we cannot determine the precise location of an electron within an atom, we can make measurements of the location of the electron. With these measurements, we find that each results in a different value for the location. Even though we can’t pin the electron down, we can determine a probability distribution for where the electron is observed.

This probability distribution is the most that we can know about the location and motion of an electron. It is extremely difficult to observe, but it can be determined by calculations from the field of quantum mechanics. The postulates (or rules) of quantum mechanics cannot be deduced from our experimental observations, and the calculations are far beyond what we need to worry about here. For what we need in this study, our observations, such as electron diffraction and the quantized energy levels for the electron in a hydrogen atom, are all consistent with the predictions of quantum mechanics. We will treat the predictions of quantum mechanics as the equivalent of experimental observations, conclusions that we can work with and build on.