In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, the atomic number is also equal to the number of electrons.

The atomic number, Z, should not be confused with the mass number, A, which is the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes), and the mass defect is usually very small compared to the mass, the atomic mass of an atom is roughly equal to A.

Atoms having the same atomic number Z but different neutron number N, and hence different atomic mass, are known as isotopes. Most naturally occurring elements exist as a mixture of isotopes, and the average atomic mass of this mixture determines the element's atomic weight.

3-2-1 Contact is an American science educational television show that aired on PBS from 1980 to 1988, and an adjoining children's magazine. The show, a production of the Children's Television Workshop, teaches scientific principles and their applications. Dr. Edward G. Atkins, who was responsible for much of the scientific content of the show, felt that the TV program wouldn't replace a classroom but would open the viewers to ask questions about the scientific purpose of things.

3-2-1 Contact was the brainchild of Samuel Y. Gibbon, Jr., who had been the executive producer of The Electric Company for CTW from 1971 to 1977. (Gibbon actually left CTW before Contact's production began, though he was still credited as "Senior Consultant.")

The first season of 65 programs began airing January 14, 1980 on select PBS member stations; it featured a cast of three college students who discussed science in an on-campus room known as the Workshop. The first season came to an end on April 11, 1980, but funds for more episodes were not sufficient until 1982.