See also: List of elements by atomic number A table of chemical elements ordered by atomic number and color coded according to type of element. ...

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, the number of electrons also equals the atomic number. Image File history File links Atomicnumber. ... For other uses, see Chemistry (disambiguation). ... A magnet levitating above a high-temperature superconductor demonstrates the Meissner effect. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... The nucleus of an atom is the very small dense region, of positive charge, in its centre consisting of nucleons (protons and neutrons). ... For other uses, see Atom (disambiguation). ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... Properties The electron (also called negatron, commonly represented as e−) is a subatomic particle. ...

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 mass number defines the isotope of the element and often comes after the name of the element, e.g. carbon-14 (used in carbon dating). The mass number (A), also called atomic mass number (not to be confused with atomic number (Z) which denotes the number of protons in a nucleus) or nucleon number, is the number of nucleons (protons and neutrons) in an atomic nucleus. ... This article or section does not adequately cite its references or sources. ... For other uses, see Isotope (disambiguation). ... Carbon-14 is the radioactive isotope of carbon discovered February 27, 1940, by Martin Kamen and Sam Ruben. ... Radiocarbon dating is the use of the naturally occurring isotope of carbon-14 in radiometric dating to determine the age of organic materials, up to ca. ...

History

The atomic number originally meant the number of an element's place 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 Periodic Table” redirects here. ... Portrait of Dimitri Mendeleyev by Ilya Repin Dimitri Mendeleev (Russian: , Dimitriy Ivanovich Mendeleyev  ) (8 February [O.S. 27 January] 1834 in Tobolsk – 2 February [O.S. 20 January] 1907 in Saint Petersburg), was a Russian chemist. ... ... For the record label, see Iodine Recordings. ... General Name, Symbol, Number tellurium, Te, 52 Chemical series metalloids Group, Period, Block 16, 5, p Appearance silvery lustrous gray Standard atomic weight 127. ...

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. Henry Moseley at work. ... Year 1913 (MCMXIII) was a common year starting on Wednesday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Tuesday of the 13-day-slower Julian calendar). ... X-ray crystallography is a technique in crystallography in which the pattern produced by the diffraction of x-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ...

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. Electron atomic and molecular orbitals In atomic physics and quantum chemistry, the electron configuration is the arrangement of electrons in an atom, molecule, or other physical structure (, a crystal). ... For a less technical and generally accessible introduction to the topic, see Introduction to quantum mechanics. ... Example of a sodium electron shell model An electron shell, also known as a main energy level, is a group of atomic orbitals with the same value of the principal quantum number n. ... In chemistry, a chemical bond is the force which holds together atoms in molecules or crystals. ...

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. Half-Life For a quantity subject to exponential decay, the half-life is the time required for the quantity to fall to half of its initial value. ... 3-dimensional rendering of the theoretical Island of Stability. ...

See also

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

The periodic table is a tabular method of displaying the chemical elements. ... Effective atomic number has two different meanings: one that is the effective nuclear charge of an atom, and one that calculates the average atomic number for a compound or mixture of materials. ... In chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms, as opposed to obsolete beliefs that matter could be divided into any arbitrarily small quantity. ... Prouts hypothesis was an early 19th century attempt to explain the existence of the various chemical elements through a hypothesis regarding the internal structure of the atom. ...

Notes