The chemical bonds that link atoms in molecules can change in a chemical reaction, a process by which atoms or molecules, called reactants, are transformed into different molecules, called products. During a chemical reaction, atoms keep their identity but change which atoms they are bonded to.

For example, two molecules of hydrogen gas (2H₂) and one molecule of oxygen gas (O₂) can react to form two molecules of water (2H₂O), as shown in Fig. 2.9. In this reaction, the numbers of each type of atom are conserved, but their arrangement is different in the reactants and the products. Specifically, the H–H bond in hydrogen gas and the O=O bond in oxygen are broken. At the same time, each oxygen atom forms new covalent bonds with two hydrogen atoms, forming two molecules of water. In fact, this reaction is the origin of the name “hydrogen,” which literally means “water former.” The reaction releases a good deal of energy; it was used in the main engine of the space shuttle.

In biological systems, chemical reactions provide a way to build and break down molecules for use by the cell, as well as to harness energy, which can be held in chemical bonds (Chapter 6).