The Action of Molecules Defines Life

Many of the details of life can be understood by considering the behavior of molecules. For example, antibiotics are molecules, and their targets are molecules; antibiotic resistance arises through changes in molecules. Because atoms are the building blocks of molecules, our discussion of molecules must begin with atoms.

Atoms are tiny particles that exist in about 100 different types called elements. Familiar elements are oxygen, hydrogen, carbon, iron, and zinc. Each atom contains a nucleus composed of smaller particles called protons and neutrons. The number of protons in the nucleus defines the element. For example, hydrogen atoms have one proton; oxygen has sixteen. Each atom also contains a surrounding cloud of electrons. Protons behave as if they are positively charged, whereas electrons are considered to be negatively charged. An atom in its elemental form contains the same number of protons and electrons. Such an atom is electrically neutral.