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 conventionally 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 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.
History
Loosely speaking, the existence of a periodic table creates an ordering for the elements.
Such an ordering is not necessarily a numbering, but can be used to construct a numbering by fiat.
Dmitri Mendeleev said he arranged his tables in order of atomic weight ("Atomgewicht")[>The Periodic Table of Elements (American Institute of Physics)] However, in deference to the observed chemical properties, he violated his own
rule and placed tellurium ahead of iodine.[>The Development of the Periodic Table (Chemsoc)] This placement is consistent with the modern practice of ordering the elements by proton number, Z.
A numbering based on the periodic table was never entirely satisfactory. For one thing,
the gradual identification of more and more lanthanoids over many decades led to
long-term instability in the numbering of all elements from hafnium on up.
The situation improved dramatically after research by Henry Moseley in 1913.[Ordering the Elements in the Periodic Table (Chemsoc)] Moseley discovered a strict relationship between the x-ray diffraction spectra of elements and their most logical location in the periodic table. This led to the conclusion that the atomic number corresponds to the electric charge of the nucleus, i.e. the proton number Z. Among other things, Moseley demonstrated that the lanthanoid series (from lanthanum to lutetium inclusive) must have 15 members -- no less and no more -- which was far from obvious from the
chemistry at that time.
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 shells, 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
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