{{seealso|List of elements by atomic number}}
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In [[chemistry]] and [[physics]], the '''atomic number''' (also known as the '''proton number''') is the number of [[proton]]s found in the [[atomic nucleus|nucleus]] of an [[atom]]. It is traditionally represented by the symbol '''Z'''. The atomic number uniquely identifies a [[chemical element]]. In an atom of [[electric charge|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 [[neutron]]s in the nucleus of an atom.

==History==
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.<ref>[http://www.aip.org/history/curie/periodic.htm The Periodic Table of Elements] (American Institute of Physics)</ref> 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.<ref>[http://www.chemsoc.org/networks/LearnNet/periodictable/pre16/develop/mendeleev.htm The Development of the Periodic Table] (Chemsoc)</ref> 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 Moseley|Henry Gwyn Jeffreys Moseley]] in 1913.<ref>[http://www.chemsoc.org/networks/LearnNet/periodictable/pre16/order/atomicnumber.htm Ordering the Elements in the Periodic Table] (Chemsoc)</ref> 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 &mdash; the charge of the protons. The atomic number is the number of protons that is equal to the number of electrons.

==Chemical properties==
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 shell]]s, particularly the [[valence shell]], is the primary factor in determining its [[chemical bonding]] behavior.

==New elements==
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.

==See also==
*[[History of the periodic table]]
*[[Effective atomic number]]
*[[Atomic Theory]]
*[[Prout's hypothesis]]

==Notes==
{{reflist}}

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[[Category:Chemical properties]]
[[Category:Nuclear physics]]

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