0.6 Electron orbitals and electron configurations in atoms  (Page 2/10)

Observation 1: electron waves and the uncertainty principle

Our first step in discovering how electrons move requires us to examine the results of an experiment seemingly far removed from the questions we are asking. These results concern something called “diffraction,” which happens when two waves collide. When two particles collide, they may bounce off of each other, stick together, or cause each other to change paths. A wave collision is perhaps hard to think about, since waves don’t exist in a single location. For this reason, we sometimes think instead of the overlap of two waves which come across each other. When two waves collide or overlap, their motions “interfere” with one another, meaning that the waves can add together or subtract from one another. This interference can be either constructive or destructive, depending upon how the waves add together. If the high points of both waves coincide in the same place, then the waves add together to give a bigger wave with a greater amplitude. If the high point of one wave adds to the low point of the other wave, then the waves cancel each other out. We can have a wave with smaller amplitude, and in some locations the interference results in zero amplitude for the wave. This is called a “node.” It is easy to see these kinds of wave interference in water waves when the waves hit a barrier and bounce back. The wave coming to the barrier and the wave leaving the barrier interfere with each other, and a beautiful pattern of high points and low points emerges. [link] is an example of a “diffraction pattern.”

A common way to observe interference with waves is to allow the wave to encounter an obstacle. For a light wave, this could be a small slit. Different pieces of the light wave encounter the slit at different points, deflecting in varying directions rather than going straight through the slit. When light is passed through a series or grid of small slits, the deflected light pieces can then interfere with one another either constructively or destructively, depending upon the angle at which the light approaches the grid. Since we can get both increased and decreased amplitude, we can see a beautiful diffraction pattern, just like water waves and as seen in [link] . Since the grid can produce a diffraction pattern, it is called a diffraction grating.

The comparison between the diffraction patterns of water waves and of light is very strong evidence to prove to us that light moves like a wave. Recall, however, that our earlier conclusion is that light behaves as a collection of energy packets , or photons. This means that light has some characteristics which are like particles and some which are like waves.

Up to this point, we have assumed that electrons are simply particles, behaving essentially as billiard balls or planets. To test this assumption, we try reflecting electrons off of a surface of a metal and looking at the pattern produced when the electrons return from their interaction with the surface. Since the metal consists of atoms, the metal surface looks to the incoming electron like a diffraction grating, with grooves spaced one atom apart. As we can see in [link] , we observe in this experiment that the reflected electrons produce a pattern very similar to that observed by diffracted light. Certain angles of incidence and reflection produce no reflected electrons. These angles are alternated with angles with strong probability for reflection of electrons. This is very strong evidence that electrons move as waves!