Atomic number

From Example Problems
Jump to navigation Jump to search

The atomic number (Z) is a term used in chemistry and physics to represent the number of protons found in the nucleus of an atom. In an atom of neutral charge, the number of electrons equals the atomic number.

The atomic number originally meant the number of an element's place in the periodic table. When Mendeleev arranged the known chemical elements grouped by their similarities in chemistry, it was noticeable that placing them 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).

See also

af:Atoomgetal als:Ordnungszahl ar:رقم ذري bg:Атомен номер br:Niver atomek ca:Nombre atòmic cs:Atomové číslo da:Atomnummer de:Ordnungszahl et:Järjenumber es:Número atómico eo:Atomnumero fr:Numéro atomique ko:원자 번호 io:Atomala nombro id:Nomor atom it:Numero atomico he:מספר אטומי lt:Atomo numeris nl:Atoomnummer ja:原子番号 no:Atomnummer nn:Atomnummer pl:Liczba atomowa pt:Número atómico ru:Атомный номер sk:Protónové číslo sl:Vrstno število fi:Järjestysluku (kemia) sv:Atomnummer th:เลขอะตอม zh:原子序数