Pursuing a Metals Theory

Abstract

Scientists, following the dictum not to make more hypotheses than they need, like to keep their theoretical models simple. Unnecessary complications are not popular. But when too many facts fall outside the model, it has to be expanded. The theoretical picture of sodium, potassium and other alkali metals has reached that point, says solid-state physicist Albert W. Overhauser, of the Scientific Laboratories at Ford Motor Co., who proposes a new, more complex model. If the new model is accurate, a tremendous body of theoretical work on the properties of metals will have to be revised. Only in the last few decades has it been possible to describe the details of how atoms in a solid behave. Out of this understanding has come, among other technological advances, the development of transistors and other semiconducting devices. Although the alkali metals are not as technologically usefuLl in their pure state as other metals-they react too quickly with other elements their simple atomic structure has made them basic to the understanding of other more complicated elements. Their distinguishing feature is that they have only one, easily-detachable electron in the ouLter shell, and few electrons in inner shells. From this, it has appeared possible to assume that the outer electrons are free to move about within the solid, forming a sort of gas, instead of being tied down to orbits around particular atoms. As in other condulcting metals, it is this gas of free electrons that carries electric current. The simple model of the alkalis assumes that the sum of all forces on any one outer-shell electron is zero; that is, the forces cancel out, and the electrons move about unaffected, on average, by the atomic nuclei. A result of this is that the density of the electron gas is the same throughout the metal. From this simple model, the properties of the metals can be predicted. Physicists can measure what happens to the metals when they are bathed in light, placed in magnetic fields, or subjected to electric forces. Similar quantities can be measured for more complex elements, too-elements like copper, iron and chromium whose theoretical models are more complicated. Differences in measured behavior of simple and complex metals can give clues as to the structure of those more complicated elements. In this way, the simple theory helps experimenters to choose what measurements to make, and the experiments elaborate the theory. The trouble is, according to Dr. Overhauser, that experiments with alkali metals have turned up results different from those predicted by the simple model. The amount of light energy, for instance, that is absorbed by a metal, compared with what it reflects, depends on the frequency of the incoming light. The absorption pattern of metals is a commonly measured quantity. One of the most important misbehaviors of the alkali metals is the way they absorb light. According to the simple theory, absorption by potassium should decrease smoothly with increasing frequency up to 1.3 electron volts. But an experiment four years ago in Germany showed a big peak in the absorption curve at 0.6 electron volt. Other effects, mostly concerned with the magnetic behavior of the metals, have been observed to vary from what (see p. 380) Dr. Overhauser and a potassium sample: The simple metals are not so simple.