12: Chemical Principles

INTRODUCTION

Chemistry is the science of matter; it encompasses the structure, physicalproperties, and reactivities of atoms and their compounds. In many respects, toxicology is the science of the interactions of matter with living entities. Chemistry and toxicology are therefore intimately linked. The study of the principles of inorganic, organic, and biologic chemistry offers important insight into the mechanisms and clinical manifestations of xenobiotics and poisoning. This chapter reviews many of these tenets and provides relevance to the current practice of medical toxicology.

THE STRUCTURE OF MATTER

Basic Structure

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 it 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 (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. 134). The atomic weight, measured in grams per mole (g/mol), also indicates the molar mass of the element. That is, in one atomic weight (12.011 g for ­carbon) there is one mole (6.023 × 10²³) of 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 be separated only by chemical means that destroy the original compound, as occurs during the burning (ie, oxidation) of a hydrocarbon, a process that releases the carbon principally as carbon dioxide and the hydrogen principally as water (H₂O). This important property differentiates compounds from mixtures, which are combinations of elements or compounds that can be separated by physical means such as the distillation of petroleum into its hydrocarbon components or the evaporation of seawater to separate water from sodium chloride and other substances. With notable exceptions, such as the elemental forms of many metals or halogens ...