Atomic number

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In chemistry and physics, the atomic number (Z) (also known as the proton number) is the number of protons found in the nucleus of an atom. In an atom of neutral charge, the number of electrons also equals the atomic number.

The atomic number originally meant the number of an element's place in the periodic table. When Dmitri Mendeleev arranged the known chemical elements grouped by their similarities in chemistry the list was missing the noble gases. It was also noticeable that placing the elements in strict order of atomic mass resulted in some mismatches. Iodine and tellurium, if listed by atomic mass, appeared to be in the wrong order, and would fit better if their places in the table were swapped. Placing them in the order which fit chemical properties most closely, their number in the table was their atomic number. This number appeared to be approximately proportional to the mass of the atom but, as the discrepancy showed, reflected some other property than mass.

The anomalies in this sequence were finally explained after research by Henry Gwyn Jeffreys Moseley in 1913. Moseley discovered a strict relationship between the x-ray diffraction spectra of elements, and their correct location in the periodic table. It was later shown that the atomic number corresponds to the electric charge of the nucleus — in other words the number of protons. It is the charge which gives elements their chemical properties, rather than the atomic mass.

The atomic number is closely related to the mass number (although they should not be confused) which is the number of protons and neutrons in the nucleus of an atom. The mass number often comes after the name of the element, e.g. carbon-14 (used in carbon dating).

[edit] See also

als:Ordnungszahl ar:رقم ذري ast:Númberu atómicu zh-min-nan:Goân-chú-hoan bs:Atomski broj br:Niver atomek bg:Атомен номер ca:Nombre atòmic cs:Atomové číslo da:Atomnummer de:Ordnungszahl et:Järjenumber el:Ατομικός αριθμός es:Número atómico eo:Atomnumero eu:Zenbaki atomiko fr:Numéro atomique ga:Uimhir adamhach gl:Número Atómico ko:원자 번호 hr:Atomski broj io:Atomala nombro id:Nomor atom is:Sætistala it:Numero atomico he:מספר אטומי lb:Uerdnungszuel lt:Atomo numeris lmo:Nümer atòmich mk:Атомски број nl:Atoomnummer ja:原子番号 no:Atomnummer nn:Atomnummer nds:Atomtall pl:Liczba atomowa pt:Número atómico ro:Număr atomic ru:Зарядовое число simple:Atomic number sk:Protónové číslo sl:Vrstno število sr:Атомски број sh:Atomski broj fi:Järjestysluku (kemia) sv:Atomnummer ta:அணு எண் th:เลขอะตอม vi:Số nguyên tử tr:Atom numarası uk:Атомний номер uz:Atom raqami zh:原子序数

Atomic number

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