Chemistry is the science of matter; it encompasses the structure, physical properties, and reactivities of atoms and their compounds. In many respects, toxicology is the science of the interactions of matter with physiologic entities. Chemistry and toxicology thereby are intimately linked. The study of the principles of inorganic, organic, and biologic chemistry offer important insight into the mechanisms and clinical manifestations of xenobiotics and poisoning, respectively. This chapter reviews many of these tenets and provides relevance to the current practice of medical toxicology.
Matter includes the substances of which everything is made. Elements are the foundation of matter, and all matter is made from one or more of the known elements. An atom is the smallest quantity of a given element that retains the properties of that element. Atoms consist of a nucleus, incorporating protons and neutrons, coupled with its orbiting electrons. The atomic number is the number of protons in the nucleus of an atom, and is a whole number that is unique for each element. Thus, elements with 6 protons are always carbon, and all forms of carbon have exactly 6 protons. However, although the vast majority of carbon nuclei have 6 neutrons in addition to the protons, accounting for an atomic mass (i.e., protons plus neutrons) of 12 (12C), a small proportion of naturally occurring carbon nuclei, called isotopes, have 8 neutrons and a mass number of 14 (14C). This is the reason that the atomic weight of carbon displayed on the periodic table is 12.011, and not 12, as it actually represents the average atomic masses of all isotopes found in nature weighted by their frequency of occurrence. Moreover, 14C is actually a radioisotope, which is an isotope with an unstable nucleus that emits radiation (particles or rays) until it achieves a stable state (Chap. 133). The atomic weight, measured in grams per mole (g/mol), also indicates the molar mass of the element. That is, in 1 atomic weight (12.011 g for carbon) there is 1 mole of atoms (6.023 × 1023 atoms).
Elements combine chemically to form compounds, which generally have physical and chemical properties that differ from those of the constituent elements. The elements in a compound can only be separated by chemical means that destroy the original compound, as occurs during the burning (ie, oxidation) of a hydrocarbon, which releases the carbon as carbon dioxide. This important property differentiates compounds from mixtures, which are combinations of elements or compounds that can be separated by physical means. For example, this occurs during the distillation of petroleum into its hydrocarbon components or the evaporation of seawater to leave sodium chloride. With notable exceptions, such as the elemental forms of many metals or halogens (eg, Cl2), most xenobiotics are compounds or mixtures.
Dimitri Mendeleev, a Russian chemist in the mid-19th century, recognized that ...