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.

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.