An element is a pure substance that contains only one kind of atom. The element hydrogen consists only of hydrogen atoms; the element iron consists only of iron atoms. The atoms of each element have certain characteristics or properties that distinguish them from the atoms of other elements. These physical and chemical properties (how they interact with other atoms) depend on the numbers of particles the atoms contain.
There are 94 elements in nature, and at least another 24 have been made in physics laboratories. About 98 percent of the tissue of every living organism (except in skeletons) is composed of just six elements:
| Carbon (symbol C) | Hydrogen (H) | Nitrogen (N) |
| Oxygen (O) | Phosphorus (P) | Sulfur (S) |
The biological roles of these elements will be our major concern in this book, but other elements are found in living organisms as well. Sodium and potassium, for example, are essential for nerve function; calcium can act as a biological signal; iodine is a component of a vital hormone; and magnesium is bound to chlorophyll in plants.