CHAPTER FIVE QUANTUM QUESTIONS While chopping onions or producing sawdust in a woodwork project, have you ever wondered what effect making smaller and smaller pieces would have? Our science courses teach us that the materials we see are made up of molecules formed from combinations of atoms, and each different combination has different properties. In the Golden Age of Greece, Leucippus, a student of Zeno in the school of Pythagoras, proposed that the world consisted entirely of atoms with empty space in between. Most Greek thinkers believed that each material was a particular blend of the four elements earth, air, fire, and water. After Democritus went on to explain how different atoms would provide different observed properties in materials, little further expansion of the atomic theory of materials took place for over 2000 years until John Dalton put forth his atomic theory at the start of the 19th century. If we leap ahead to the start of the 20th Century, we find that the results observed by the chemists like Dalton and Joseph Priestly over the previous two centuries, plus new instruments available to physicists, began to divulge the properties that an atom must exhibit. Ionized gases, such as those in a mercury or sodium arc lamp, were seen to emit not a continuous spectrum of light, but rather a series of discrete, well-defined colors of light. In 1911 Lord Rutherford observed that the positive electrical charge is concentrated in a very small, relatively massive nucleus of an atom, and this nucleus is surrounded by a cloud of light-weight, negatively-charged electrons, the total charge being balanced in an atom. This 'solar system model' of the atom was revolutionary for the time. Niels Bohr, in collaboration with Rutherford, developed a model for an atom that required that the electrons move in circular orbits around the nucleus, orbits that have certain quantized amounts of momentum. Many such orbits exist, but the electrons tend to fill the lowest-energy orbits, with only a few allowed to occupy each level. If an electron is caused to undergo a transition from one orbit to another, it must absorb or emit a wave having the energy difference of those orbits. These wave-like emissions consist of visible light or X-ray radiation. Transitions between the allowed orbits in an atom thus cause radiations having energies associated with the energy differences of the allowed orbits. We see that the discrete colors of light that are seen in gas arc lamps can be understood as the radiations corresponding to electrons, previously excited by electric forces, undergoing transitions from one higher-energy allowed orbit to another lower-energy one. The success of Bohr's model for the atom in explaining the discrete colors emitted by gaseous arc lamps brought immediate attention to the question why this structure should exist and be stable. The theory of quantum mechanics was developed to describe these phenomena. Not only has quantum theory been successful in describing atomic structure and the interactions with radiations, but it has also been applied to understand the structure of the nucleus and radiations and particles associated with that part of an atom. So we see that, just as you might wonder how small you can chop the onion and how the properties would change as the pieces shrink, the scientists whose unexplained observations of the light emissions from atoms were inspired to completely change their way of thinking about matter. The concepts they developed for the very small world of atoms, the quantum world, get "curiouser and curiouser." Learn more about these concepts and the people who developed them by going to the web sites cited below. http://www.physics.gla.ac.uk/introPhy/Famous/rutherford/rutherford.html http://www.orcbs.msu.edu/radiation/radhistory/nielsbohr.html http://www.anu.edu.au/physics/courses/Physics2000/quantumzone/index.html Check back soon for the Complete Story! Last Updated: