, 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 different than the atomic weight of a element, because in general an element consists of a mixture of atoms with the same Z but differing N (isotopes).
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") However, in deference to the observed chemical properties, he violated his own
rule and placed tellurium
ahead of iodine
. This placement is consistent with the modern practice of ordering the elements by proton number, Z
A numbering based on the periodic table was never entirely satisfactory. For one thing,
the gradual identification of more and more lanthanoids over many decades led to
long-term instability in the numbering of all elements from hafnium on up.
The situation improved dramatically after research by Henry Moseley in 1913. 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 lanthanoid series (from lanthanum to lutetium inclusive) must have 15 members -- no less 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 protons in its nucleus. The charge of an atom's nucleus defines its electron configuration
based on principles of quantum mechanics
. The form of each element's electron shells
, particularly the valence shell
, is the primary factor in determining its chemical bonding
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.