or as complex as the electromagnetic spectrum of a star. Of course, it is possible to measure both the dip of rock strata and a stellar spectrum incorrectly. Before an observation becomes part of the body of scientific knowledge, we would like some reassurance that it is right. How can we tell whether observations are right or not? The most important way to verify an observation is to replicate it independently. In the strictest sense, independent means by a separate observer, team of observers, or laboratory, and preferably by a different technique or instrument. It is not practicable to replicate every observation in this manner, but critical observations, those which appear to be inconsistent with existing theories or which test the predictions of newly established ones should be, and generally are, replicated. But even replication does not guarantee that an observation is correct.
Observations form the basis of theories. Theories are also called models, hypotheses, or laws. Scientific understanding is achieved by constructing and modifying theories to explain observations. Theories are merely the products of the imagination of scientists, so we also need a method of sorting out “correct” theories from “incorrect” ones. Good theories not only explain existing observations, but also make predictions about the outcome of still unperformed experiments or observations. Theories are tested by performance of these experiments and comparison of the results with the predictions of the theory. If the predictions are correct, the theory is accepted and the phenomenon considered to be understood, at least until a new and different test is performed. If the predictions are incorrect, the theory is discarded or modified. When trying to explain a newly discovered phenomenon, scientists often reject many new theories before finding a satisfactory one. But long-standing theories that successfully explain a range of phenomena can often be modified without rejecting them entirely when they prove inconsistent with new observations. And as Carl Sagan once said, extraordinary claims require extraordinary proof.
Occasionally, new observations are so inconsistent with a well-established theory that it must be discarded entirely and a new one developed to replace it. Scientific “revolutions” occur when major theories are discarded in this manner. Rapid progress in understanding generally accompanies these revolutions. Such was the case in physics in the early twentieth century when the quantum and relativity theories supplanted Newtonian theories (Lindley, 2001). The development of plate tectonics in the 1960s and 1970s is an excellent example of a scientific revolution in which old theories were replaced by a new unifying one. A range of observations including the direction of motion along transform faults, the magnetic anomaly pattern on the sea floor, and the distribution of earthquakes and volcanoes were either not predicted by, or were inconsistent with, classical theories of the Earth. Plate tectonics explained all these and made a number of predictions, such as the age of the sea floor, that could be tested. Thus scientific understanding progresses through an endless cycle of observation, theory construction and modification, and prediction.
In this cycle, theories can achieve acceptance, but can never be proven correct, because we can never be sure that it will not fail some new, future test.
Quite often, it is possible to explain observations in more than one way. That being the case, we need a rule that tells us which theory to accept. When this occurs, the principle is that the theory that explains the greatest range of phenomena in the simplest manner is always preferred. For example, the motion of the Sun across the sky is quite simple and may be explained equally well by imagining that the Sun orbits the Earth as vice versa. Ancient astronomers could, for example, predict eclipses. However, the motions of the planets in the sky are quite complex and require a very complex theory if we assume that they orbit the Earth. If we theorize that the Earth and the other planets all orbit the Sun, the motions of the planets become simple elliptical orbits and can be explained by Newton's three laws of motion.† The geocentric theory was long ago replaced by the heliocentric one for precisely this reason. This principle of simplicity, or elegance, also applies to mathematics. Computer programmers call it the KISS (Keep It Simple, Stupid!) principle. In science, we call it parsimony, and can sum it up by saying: Don't make nature any more complex than it already is.
1.4.2 The scientist as skeptic
Although we often refer to scientific facts, there are no facts in science. A fact, by definition, cannot be wrong. Both observations and theories can be, and sometimes are, wrong. Of course, some observations (e.g., the Sun rises each morning in the East) and theories (the Earth revolves around the Sun) are so oft-repeated and so well established that they are not seriously questioned. But remember that the theory that the Sun revolves around the Earth was itself once so well established that Galileo was tried and sentenced to house arrest for questioning it.
One of the ways in which science differs from other fields of endeavor is that in science, nothing is sacred. It is best to bear in mind the possibility, however remote, that any observation or theory can be wrong. Conversely, we must also accept the possibility that even the wildest observations and theories might be correct: in quantum physics, for example, there is a great range of well-replicated observations that can only be labeled as bizarre (see, e.g., Gribbin, 1984). “Intuition” plays a greater role in science than most scientists might be willing to admit, even though scientific intuition is often very useful. Nevertheless, our intuition is based largely on our everyday experience, which is very limited compared with the range of phenomena that science attempts to understand. As a result, our intuition often deceives us. Sometimes we must put it aside entirely. That a clock will run slower if it moves faster, or that an electron can behave as both a wave and a particle, or that continents move great distances, are all very counter-intuitive observations, but all are (apparently) correct. Thus, skepticism is one of the keys to good science. In science, never totally believe anything, but never totally disbelieve anything, either.
1.5 ELEMENTS, ATOMS, CRYSTALS, AND CHEMICAL BONDS: SOME CHEMICAL FUNDAMENTALS
1.5.1 The periodic table
We'll begin our very brief review of chemical fundamentals with the periodic table (Figure 1.1). In Dmitri Mendeleyev's‡ day, chemistry and geochemistry were not as distinct as they are today. Chemists were still very much occupied with discovering new elements, and they sought them in natural materials. For a variety of reasons, therefore, the Mendeleyev's periodic table provides a good point of departure for us.
Mendeleyev's periodic table of the elements was the sort of discovery that produces revolutions in science. Chemistry had evolved tremendously through the first half of the nineteenth century. Between the publication of Lavoisier's The Elements of Chemistry, often considered the first modern text in chemistry, in 1789 and Mendeleyev's 1869 paper, the number of known elements had increased from 23 to 67. The concepts of the atom and the molecule were well established, and the role of electromagnetic forces in chemical interactions was at least partly understood. Nevertheless, the structure of atoms, and how this structure governed chemical properties of the atom, were to be twentieth-century discoveries (though there were some interesting prescient theories). Mendeleyev's great contribution was to show that properties of the elements are a periodic function of atomic weights. Like all good scientific theories, this one made predictions: Mendeleyev was not only able to predict the discovery of then-unknown elements, such as B, Sc, Ga, and Ge, but also their characteristics and the materials or minerals in which they were most likely to be found (Strathern, 2000). The periodic table led the way not only to the discovery of the remaining elements, but also to understanding the fundamental controls on chemical behavior.
Figure 1.1 shows the periodic table as we know it today. Like most theories, Mendeleyev's has gone through some revision since it was first proposed. Most importantly, we now organize the periodic table based on atomic number rather than atomic weight. The atomic number of an element is its most important property and is determined by the number of protons in the nucleus (thus the terms atomic number