Atomic number

Not to be confused with Atomic mass, Mass number, or Atomic weight.

An explanation of the superscripts and subscripts seen in atomic number notation. Atomic number is the number of protons, and therefore also the total positive charge, in the atomic nucleus.

The atomic number or nuclear charge number (symbol Z) 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 (n_p) 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.

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 molarity.

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 must have been he who established this definition.

After Rutherford deduced the existence of the proton in 1920, "atomic number" customarily referred to the charge number or proton number of an atom.

Some writers in the 1920s defined the atomic number as being the number of "excess protons" in the nucleus. The atomic weight of most atoms exceeds the atomic number, and scientists thought that the surplus weight was carried by protons which were paired with "nuclear electrons" in what Rutherford called "neutral doublets". Scientists abandoned the idea of nuclear electrons when neutrons were discovered in 1932.

Chemical properties

Each element has a specific set of chemical properties as a consequence of the number of electrons present in the neutral atom, which is Z (the atomic number). The configuration of these electrons follows from the principles of quantum mechanics. The number of electrons in each element's electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. Hence, it is the atomic number alone that determines the chemical properties of an element; and it is for this reason that an element can be defined as consisting of any mixture of atoms with a given atomic number.

New elements

The quest for new elements is usually described using atomic numbers. As of 2024, all elements with atomic numbers 1 to 118 have been observed. 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 of a nuclide becomes shorter as atomic number increases, though undiscovered nuclides with certain "magic" numbers of protons and neutrons may have relatively longer half-lives and comprise an island of stability.

A hypothetical element composed only of neutrons, neutronium, has also been proposed and would have atomic number 0, but has never been observed.

See also

• Atomic theory
• Chemical element – Chemical substance not composed of simpler ones
• Effective nuclear charge – Measurement in atomic physics
• Effective atomic number (compounds and mixtures) – Approximate atomic number calculated for materials with many elements
• Even and odd atomic nuclei – Nuclear physics classification method
• History of the periodic table – Development of the table of chemical elements
• List of chemical elements
• Mass number – Number of heavy particles in the atomic nucleus
• Neutron number – The number of neutrons in a nuclide
• Neutron–proton ratio – Ratio of neutrons to protons in an atomic nucleus
• Prout's hypothesis – Early model of the atom that did not account for mass defect