In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, atomic number is equal to the number of electrons.
The atomic number, Z, should not be confused with the mass number, A, which is the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes), the atomic mass of an atom is roughly equal to A. Note that the atomic mass number A of an atom, which is always an integer, is usually different from the atomic weight of a element, because (among other reasons) most elements consist of a mixture of stable atoms with the same Z but differing N (isotopes).
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Loosely speaking, the existence of a periodic table creates an ordering for the elements. Such an ordering is not necessarily a numbering, but can be used to construct a numbering by fiat. Dmitri Mendeleev said he arranged his tables in order of atomic weight ("Atomgewicht")[1] However, in deference to the observed chemical properties, he violated his own rule and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9).[1][2] This placement is consistent with the modern practice of ordering the elements by proton number, Z. (In the modern periodic table, argon (atomic weight 39.9) is also ahead of potassium (39.1), but Mendeleev's table did not include the noble gases.)
A numbering based on the periodic table was never entirely satisfactory. For one thing, the gradual identification of more and more lanthanides led to inconsistency in the numbering of all elements from hafnium on up.
The situation improved dramatically after research by Henry Moseley in 1913.[3] Moseley discovered a strict relationship between the x-ray diffraction spectra of elements and their most logical location in the periodic table. This led to the conclusion that the atomic number corresponds to the electric charge of the nucleus, i.e. the proton number Z. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members — no fewer and no more — which was far from obvious from the chemistry at that time.
Each element has a specific set of chemical properties as a consequence of the number of electrons, which in a neutral atom equals the number of protons in its nucleus. The configuration of these electrons follows from the principles of quantum mechanics. The occupancy of each element's electron shells, particularly the valence shell, is the primary factor in determining its chemical bonding behavior.
The quest for new elements is usually described using atomic number. As of early 2007, elements with atomic numbers through 118 (excluding 117) have been discovered. 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 becomes shorter as atomic number increases, though an "island of stability" may exist for undiscovered isotopes with certain numbers of protons and neutrons.