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See also: List of elements by atomic number

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. It is traditionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, atomic number is equal to the number of electrons.

The atomic number is closely related to the mass number, which is the number of protons and neutrons in the nucleus of an atom.

The atomic number originally was used to signify the element's location in the periodic table. Dmitri Mendeleev arranged the known elements in increasing order of atomic weight and grouped by their similar chemical properties.^[1] However, placing the elements in strict order of atomic weight resulted in some mismatches. Iodine and tellurium, if listed by atomic weight, appeared to be in the wrong order; and would fit better by chemical properties if their places in the table were swapped.^[2] Placing them in the order which fit their properties most closely, their number in the table was their atomic number. This number appeared to be related to the mass of the atom but, as the discrepancy showed, reflected some property other than mass.

The anomalies in this sequence were finally explained after research by Henry Gwyn Jeffreys Moseley in 1913.^[3] Moseley discovered a strict relationship between the x-ray diffraction spectra of elements, and their correct location in the periodic table. This led to the conclusion that the atomic number corresponds to the electric charge of the nucleus — the charge of the protons. The atomic number is the number of protons that is equal to the number of electrons.

Each element has a specific set of chemical properties as a consequence of the number of protons in its nucleus. The charge of an atom's nucleus defines its electron configuration based on principles of quantum mechanics. The form of each element's electron shells, particularly the valence shell, is the primary factor in determining its chemical bonding behavior.

The quest for new elements is usually described using atomic number. As of early 2007, elements with atomic numbers through 118 (excluding 117) have been discovered. Synthesis of new elements is accomplished by bombarding target atoms of heavy elements with ions, such that the sum of the atomic numbers of the target and ion elements equals the atomic number of the element being created. In general, the half-life becomes shorter as atomic number increases, though an "island of stability" may exist for undiscovered isotopes with certain numbers of protons and neutrons.

• History of the periodic table
• Effective atomic number
• Atomic Theory
• Prout's hypothesis