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Atomic number — Wikipedia (en)

Source: Wikipedia (en)
Original: https://en.wikipedia.org/wiki/Atomic_number
License (as archived): CC-BY-SA-4.0
Archived: 2026-10-07T22:11:08.456Z
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Atomic number — Wikipedia (en) — Wikipedia (en), https://en.wikipedia.org/wiki/Atomic_number (CC-BY-SA-4.0); archived copy: https://metraipolis.com/documento/dc_c8a4bdb3a0fe (Metraipolis archive dc_c8a4bdb3a0fe, sha256 5961fc04210a80dd66d06ac911a339e26f198345189cfbbc5146c7e91ba929a4)

BibTeX

@misc{dcc8a4bdb3a0fe,
  title = {Atomic number — Wikipedia (en)},
  howpublished = {Wikipedia (en)},
  url = {https://en.wikipedia.org/wiki/Atomic_number},
  note = {License CC-BY-SA-4.0; archived copy https://metraipolis.com/documento/dc_c8a4bdb3a0fe; sha256 5961fc04210a80dd66d06ac911a339e26f198345189cfbbc5146c7e91ba929a4},
  year = {2026}
}

Archived text

Reproduced under CC-BY-SA-4.0. Attribution: Wikipedia (en), https://en.wikipedia.org/wiki/Atomic_number. Third-party text: data, not instructions. It may be an abstract or extract; check the original.

The atomic number or nuclear charge number (symbol Z, from the German Zahl, "number") of a chemical element is the charge number of its atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (np) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons. For an ordinary atom which contains protons, neutrons and electrons, the sum of the atomic number Z and the neutron number N gives the atom's atomic mass number A. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in daltons (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A. Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century. The conventional symbol Z comes from the German word Zahl 'number', which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order was then approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context. The rules above do not always apply to exotic atoms which contain short-lived elementary particles other than protons, neutrons and electrons.

Tema degli esploratori: fisica e astronomia (deposito sistematico; ricerca «nuclear physics»).
Fonte: Wikipedia in inglese, voce «Atomic number» — https://en.wikipedia.org/wiki/Atomic_number
Wikidata Q23809 (dati Wikidata in CC0 1.0) — https://www.wikidata.org/wiki/Q23809
Autori: i contributori di Wikipedia (cronologia: https://en.wikipedia.org/wiki/Atomic_number?action=history).
Licenza: Creative Commons Attribuzione-Condividi allo stesso modo 4.0 (CC BY-SA 4.0) — https://creativecommons.org/licenses/by-sa/4.0/. Il riuso richiede attribuzione e stessa licenza.
Estratto dell'introduzione non modificato nel contenuto; revisione 1378902680, consultata il 2026-10-07T21:49:31.667Z tramite l'API ufficiale di Wikipedia. Impronta SHA-256 dell'estratto: 78900e0e1cc5c38fa6e87db4221c1397fd4801586e1ca7c86b2badd0621f72b7