INTRODUCTION TO CHEMISTRY
This book is designed as a brief introduction to chemistry. Our primary objective is to give you enough understanding of the fundamental concepts and language of chemistry to allow you to read and understand articles written in newspapers such as the New York Times or in magazines such as Scientific American. Of particular importance is the concept of structure. An understanding of the shapes and motions of molecules and ions and the structure of matter in bulk is crucial to the form of the world in which we live. It is also important to know something of the language of chemistry: the meaning of words such as isotope, isomer, structural formula, the mole, the rate and extent of a reaction, equilibrium, and energy.
The approach to the topics will be informal and, we hope, not intimidating. There will be, however, the occasional problem to solve. We will also provide numerous illustrations and photographs to provide you with examples of different ways to view concepts.
WHAT IS CHEMISTRY
Chemistry is frequently defined as the study of matter and the reactions that matter undergoes. Actually, physicists, geologists, and biologists also study matter, but only chemists study the reactions that matter undergoes. For example, only chemists make compounds and try to understand the reactions that produce the compounds. Indeed, a very large segment of chemists are employed by the chemical and pharmaceutical industry for the very purpose of preparing new plastics, coatings, ceramics, drugs, fillers, alloys, and so on. These synthetic chemists must first determine what reaction can be used to synthesize their target compound and then determine what conditions will optimize the yield of the compound in order to make the compound in the most cost-effective way. After the best reaction conditions have been determined, the chemist must determine how to purify the compound, and, finally, the chemist must identify it. This final process of identification usually includes not only being certain that the compound contains the right percentage of the various elements from which it is composed, but also involves the determination of the 3-dimensional structure of the compound.
Structural details are often crucial to the activity of the compound. For example, the compounds dextrophane and levorphane differ in a very subtle way. They are non-superimposable mirror images of one another in the same way that our hands are non-superimposable mirror images of one another. Yet, because of a quirk of the evolutionary process, our bodies are able to recognize this subtle difference and produce a very different response to the two compounds: levorphan is more strongly analgesic and addictive than morphine, whereas dextrophan is neither addictive nor an analgesic. Figure 1 shows the structural formulas of the two compounds (we will discuss the various types of formulas in a later section). In Figure 2, the same molecules are shown as computer generated molecular models. In part (a), ball and stick models are shown, while part (b) shows space-filling models.
Figure 1. Structural formulas for levorphan and dextrophan.
(a)
(b)
Figure 2. Molecular models of levorphan and dextrophan. (a) ball and stick models, (b) space-filling models.
When you compare Figure 1 and Figure 2 you will find that the lines in the structural formulas indicate the attachment of atoms to one another. These lines are called bonds. Some bonds are single bonds, some are double bonds, others are triple bonds. Generally the greater the number of bonds between two atoms, the stronger the attachment of the two atoms. Notice also that in the molecular model, the atoms have different colors and sizes. The colors are obviously used to distinguish one type of atom from another. Also, recognize that the distance of the atom from the reader (depth) is indicated by the size of the atom. The space-filling model is designed to give a somewhat more accurate representation of the molecule by portraying the space filled by the electrons around the atoms. Although these models are probably more realistic representations of the molecules, they are also more difficult to "read." Most chemists prefer to see the ball and stick models, but they use space-filling representations when they are interested in the spatial requirements of certain parts of a molecule.
Molecular modeling has become an important part of the arsenal of the synthetic chemist as well as the theoretical chemist. Frequently, the synthetic chemist makes use of computer modeling to identify compounds that will have certain physical properties or produce certain physiological responses.
Theoretical and physical chemists are concerned with the description of the bonding between atoms and understanding the changes in electronic structure that occur when a reaction takes place. They produce theories or models that are eventually incorporated into the body of chemistry and used by synthetic chemists to make compounds with new, and frequently useful, properties.
Chemists can also be categorized according to the traditional sub disciplines: inorganic (elements other than carbon), organic (carbon compounds), analytical (methods used to separate and identify compounds), and physical chemists. Today, there are also many other cross disciplinary areas that occupy chemists: biochemists try to understand and apply the chemistry of biological processes, materials chemists attempt to synthesize new materials such as superconductors or artificial skin, environmental chemists study the chemistry of the environment and monitor and solve environmental problems, while forensic chemists apply chemistry to the solution of crimes.
Figure 3. The disciplines of chemistry.
SCIENTIFIC METHOD
It is important to know something about the way in which new theories are produced. To illustrate this process, sometimes called the scientific method, let us follow the thoughts and actions of a scientist who happens upon the box pictured in Figure 4. The box is apparently constructed from some heavy metal and has three rather stiff ropes protruding from the holes labeled A, B, and C. Her curiosity aroused, our scientist attempts to open the box to discover its purpose and inner workings. She soon discovers, however, that she lacks the necessary tools and must be satisfied with observations made from outside the box.
Figure 4. A curious box.
An initial tug on rope C produces no apparent movement of ropes A and B. This observation triggers the thought that the box may contain three unconnected, independent ropes. This mental visualization is our scientist's first hypothesis of the nature of the interior of the box.
Now, she reasons, if this hypothesis is correct, each rope should move to its limit without affecting (moving) any of the other ropes. In order to test her hypothesis, she pulls each rope and observes the effect of that action on the other ropes. Rope C appears to move independently of the other, but the effects of ropes A and B on each other can be expressed as a mathematical law. If x represents the distance traversed by rope A, and y the distance traversed by rope B, the law becomes
x = 2y
Because of the specific interdependency of ropes A and B, the scientist now proposes the model pictured below in Figure 5. In this model, rope A is wound around a drum attached to an axle; rope B is wound around the axle itself. The circumference of the drum is greater than the circumference of the axle (in fact, 2 times as great), and rope A is therefore played out at a greater rate than rope B. Rope C remains unattached and independent.
Figure 5. A model of the interior of the box.
This model provides an explanation for all of the experimental data, and it also permits the formulation of new questions and predictions. For example: Can rope C be withdrawn from the box completely, or is it held inside by a knot at the end? Are the ends of ropes A and B attached to the drum and the axle? If the model is correct, then when rope B is withdrawn to its limit, rope A may not have reached its limit, and (assuming that the ropes are attached) a further pull on rope A may wind rope B back into the box. These questions suggest additional experiments which might never have been conceived without the help of the model.
While our scientist has spent a considerable amount of energy investigating an almost trivial problem, her approach to the problem contains many of the features of the "scientific method": experimental observation that leads to the formulation of a law, a hypothesis that leads to a model or theory, and the subsequent use of the model to design new experiments. Each hypothesis leads to a model, which may be discarded after additional experiments are performed, or, if the experiments are all consistent with the model, the model is retained until contradictory evidence is obtained.
Because of the central role of models, it is important to be cognizant of a number of their characteristics. First, the scientist usually draws on her own experiences in fashioning a theory. In our example, it might be suggested that the ropes are controlled by elves residing in the box, but our scientist has never seen an elf, nor does she believe in the existence of such creatures. On the other hand, she has seen mechanical devices such as winches that employ ropes on drums, and she has seen a spool of thread. Many models, designed to account for the behavior of matter so small that it has never been seen, are based on the behavior of macroscopic bodies, such as billiard balls, which lie within the realm of everyone's experience.
On the other hand, some models are mathematical and abstract in nature. For example, the mathematical nature of the contemporary model of the electron makes many of its features difficult to visualize. Indeed, some scientists feel that the most significant scientific discoveries occur within the realm of mathematics.
It is also important to realize that a given set of experiments and observations can usually be explained by more than one model. Our scientist could have developed a model based on gears rather than drums, and in fact there are a number of alternate models that will satisfactorily account for the behavior of the ropes. As data and observations accumulate, one of a set of equally good models may become more satisfactory than the rest, or the choice of model may be based on considerations of simplicity or symmetry or usefulness.
Finally, the fact that models may not, and very likely do not, correspond to reality cannot be overemphasized. Since the box cannot be opened, the scientist will probably never know if the box really does contain a drum and axle. When the model is intended as a picture or visualization of matter at the sub microscopic, molecular level, the problem is even more acute. Atoms cannot possibly be either billiard balls or mathematical abstractions, nor is it likely that atoms behave like billiard balls. And yet, the billiard ball model of atoms is at the heart of the determination of the structure of the nucleic acids DNA and RNA, the revelation of the genetic code, and all of its biological implications. Thus, while the correspondence between the model and reality may not be very high, the benefits of the model, the development of new experiments, the discovery of new laws of nature, and so forth - may be very great indeed.
Reference: C. Yoder, O. Retterer, M. Thomsen, and K. Hess, Interactive Chemistry, Mosby Year-Book, 1997.
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