The units of matter that form all chemical substances are called atoms. The smallest atom, hydrogen, is approximately 2.7 billionths of an inch in diameter. Each type of atom, such as carbon, hydrogen, oxygen, and so on, is called a chemical element. A one- or two-letter symbol is used as a shorthand identification for each element. Although slightly more than 100 elements exist in the universe, only 24 are known to be essential for the structure and function of the human body. The chemical properties of atoms can be described in terms of three subatomic particles—protons, neutrons, and electrons. The protons and neutrons are confined to a very small volume at the center of an atom, the atomic nucleus, whereas the electrons revolve in orbits at various distances from the nucleus. This miniature solar-system model of an atom is an oversimplification, but it is sufficient to provide a conceptual framework for understanding the chemical and physical interactions of atoms.
Each of the subatomic particles has a different electric charge: Protons have one unit of positive charge, electrons have one unit of negative charge, and neutrons are electrically neutral. Since the protons are located in the atomic nucleus, the nucleus has a net positive charge equal to the number of protons it contains. The entire atom has no net electric charge, however, because the number of negatively charged electrons orbiting the nucleus is equal to the number of positively charged protons in the nucleus.