5.8 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Atomic number | 1/3 | https://en.wikipedia.org/wiki/Atomic_number | reference | science, encyclopedia | 2026-05-05T10:51:54.272199+00:00 | kb-cron |
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.
== Notation == The atomic number is used in AZE notation, (with A as the mass number, Z the atomic number, and E for element) to denote an isotope. When a chemical symbol is used, e.g. "C" for carbon, standard notation uses a superscript at the upper left of the chemical symbol for the mass number and indicates the atomic number with a subscript at the lower left (e.g. 32He, 42He, 126C, 146C, 23592U, and 23992U). Because the atomic number is given by the element symbol, it is common to state only the mass number in the superscript and leave out the atomic number subscript (e.g. 3He, 4He, 12C, 14C, 235U, and 239U). The common pronunciation of the AZE notation is different from how it is written: 42He is commonly pronounced as helium-four instead of four-two-helium, and 23592U as uranium two-thirty-five (American English) or uranium-two-three-five (British) instead of 235-92-uranium. Various notations appear in older sources were used, such as Ne(22) in 1934, Ne22 for neon-22 (1935) or Pb210 for lead-210 (1933)
== History == In the 19th century, the term "atomic number" typically meant the number of atoms in a given volume. Modern chemists prefer to use the concept of molar concentration. In 1913, Antonius van den Broek proposed that the electric charge of an atomic nucleus, expressed as a multiplier of the elementary charge, was equal to the element's sequential position on the periodic table. Ernest Rutherford, in various articles in which he discussed van den Broek's idea, used the term "atomic number" to refer to an element's position on the periodic table. No writer before Rutherford is known to have used the term "atomic number" in this way, so it was probably he who established this definition. After Rutherford deduced the existence of the proton in 1920, "atomic number" customarily referred to the proton number of an atom. In 1921, the German Atomic Weight Commission based its new periodic table on the nuclear charge number and in 1923 the International Committee on Chemical Elements followed suit.
=== The periodic table and a natural number for each element ===
The periodic table of elements creates an ordering of the elements, and so they can be numbered in order. Dmitri Mendeleev arranged his first periodic tables (first published on March 6, 1869) in order of atomic weight ("Atomgewicht"). However, in consideration of the elements' observed chemical properties, he changed the order slightly and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9). This placement is consistent with the modern practice of ordering the elements by proton number, Z, but that number was not known or suspected at the time. A simple numbering based on atomic weight position was never entirely satisfactory. In addition to the case of iodine and tellurium, several other pairs of elements (such as argon and potassium, cobalt and nickel) were later shown to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties. However the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from lutetium (element 71) onward (hafnium was not known at this time).
=== The Rutherford-Bohr model and van den Broek ===