Atomic number
In
chemistry and
physics, the
atomic number (
Z) (
also known as the proton number) is the number of
protons found in the nucleus of an
atom. In an atom of
neutral charge, the number of
electrons also equals the atomic number.
The atomic number originally meant the number of an element's place in the
periodic table. When
Dmitri Mendeleev arranged the known
chemical elements grouped by their similarities in chemistry, it was noticeable that placing them in strict order of
atomic mass resulted in some mismatches.
Iodine and
tellurium, if listed by atomic mass, appeared to be in the wrong order, and would fit better if their places in the table were swapped. Placing them in the order which fit chemical properties most closely, their number in the table was their atomic number. This number appeared to be approximately proportional to the mass of the atom but, as the discrepancy showed, reflected some other property than mass.
The anomalies in this sequence were finally explained after research by
Henry Gwyn Jeffreys Moseley in
1913. Moseley discovered a strict relationship between the
x-ray diffraction spectra of elements, and their correct location in the periodic table. It was later shown that the atomic number corresponds to the
electric charge of the nucleus — in other words the number of protons. It is the charge which gives elements their chemical properties, rather than the atomic mass.
The atomic number is closely related to the
mass number (although they should not be confused) which is the number of protons and
neutrons in the nucleus of an atom. The mass number often comes after the name of the element, e.g.
carbon-14 (used in
carbon dating).
*
Periodic table*
List of elements by number*
Effective atomic number*
Atomic Theory
