Atomic number

proton numberZatomic numbershigh-Znumber of protonsatomic number (Z)atomic number, Zeffective ZHigh-''Zhigh-atomic weight
The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of every atom of that element.wikipedia
638 Related Articles

Chemical element

elementelementschemical elements
The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of every atom of that element.
A chemical element is a species of atom having the same number of protons in their atomic nuclei (that is, the same atomic number, or Z).

Atom

atomsatomic structureatomic
The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of every atom of that element.
The number of protons in the nucleus, called the atomic number, defines to which chemical element the atom belongs.

Neutron number

Nnumber of neutronsneutron excess
The sum of the atomic number Z and the number of neutrons N gives the mass number A of an atom.
Atomic number (proton number) plus neutron number equals mass number: Z + N = A.

Charge number

It is identical to the charge number of the nucleus.
Atomic numbers (Z) are a special case of charge numbers, referring to the charge number of an atomic nucleus, as opposed to the net charge of an atom or ion.

Isotope

isotopesisotopicisotopic composition
Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes.
The number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral (non-ionized) atom.

Periodic table

periodic table of elementsperiodic table of the elementsperiodic system
The conventional symbol Z comes from the German word Zahl meaning 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 is approximately, but not completely, consistent with the order of the elements by atomic weights.
The periodic table, also known as the periodic table of elements, is a tabular display of the chemical elements, which are arranged by atomic number, electron configuration, and recurring chemical properties.

Mass number

atomic mass numbernucleon numberA
The sum of the atomic number Z and the number of neutrons N gives the mass number A of an atom.
Hence, the difference between the mass number and the atomic number Z gives the number of neutrons (N) in a given nucleus: N = A - Z.

Tellurium

Tenative telluriumtelluride
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).
Tellurium is a chemical element with the symbol Te and atomic number 52.

Iodine

II 2 iodinated
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).
Iodine is a chemical element with the symbol I and atomic number 53.

Lanthanide

lanthanideslanthanoidlanthanide series
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). 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.
The lanthanide or lanthanoid series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium.

Lutetium

LuLutetium orthosilicate 176 Lu
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). 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.
Lutetium is a chemical element with the symbol Lu and atomic number 71.

Hafnium

HfCeltiumhalfnium
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).
Hafnium is a chemical element with the symbol Hf and atomic number 72.

Atomic mass

atomicatomic weightmass
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 nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.
Additionally, the neutron count (neutron number) may then be derived by subtracting the number of protons (atomic number) from the mass number (nucleon count).

Henry Moseley

Henry Gwyn Jeffreys MoseleyHenry G. J. MoseleyH. G. J. Moseley
The experimental position improved dramatically after research by Henry Moseley in 1913.
Henry Gwyn Jeffreys Moseley (23 November 1887 – 10 August 1915) was an English physicist, whose contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic number.

Lanthanum

La57La 3+
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.
Lanthanum is a chemical element with the symbol La and atomic number 57.

Antonius van den Broek

Van den Broek
Nevertheless, in spite of Rutherford's estimation that gold had a central charge of about 100 (but was element Z = 79 on the periodic table), a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the central charge and number of electrons in an atom was exactly equal to its place in the periodic table (also known as element number, atomic number, and symbolized Z).
Antonius Johannes van den Broek (4 May 1870, Zoetermeer – 25 October 1926, Bilthoven) was a Dutch amateur physicist notable for being the first who realized that the number of an element in the periodic table (now called atomic number) corresponds to the charge of its atomic nucleus.

Hydrogen

HH 2 hydrogen gas
It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more than twice as much mass as would be expected from their being made of hydrogen nuclei, and thus there was required a hypothesis for the neutralization of the extra protons presumed present in all heavy nuclei.
Hydrogen is the chemical element with the symbol H and atomic number 1.

Moseley's law

announced a lawformula
This led to the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i.e. the element number Z.
It is historically important in quantitatively justifying the conception of the nuclear model of the atom, with all, or nearly all, positive charges of the atom located in the nucleus, and associated on an integer basis with atomic number.

Rutherford model

Atom symbolplanetary modelmodel
In 1911, Ernest Rutherford gave a model of the atom in which a central core held most of the atom's mass and a positive charge which, in units of the electron's charge, was to be approximately equal to half of the atom's atomic weight, expressed in numbers of hydrogen atoms.
For gold, this mass number is 197 (not then known to great accuracy) and was therefore modeled by Rutherford to be possibly 196 u. However, Rutherford did not attempt to make the direct connection of central charge to atomic number, since gold's "atomic number" (at that time merely its place number in the periodic table) was 79, and Rutherford had modeled the charge to be about +100 units (he had actually suggested 98 units of positive charge, to make half of 196).

Proton

protonsH + p
The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of every atom of that element. It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more than twice as much mass as would be expected from their being made of hydrogen nuclei, and thus there was required a hypothesis for the neutralization of the extra protons presumed present in all heavy nuclei.
The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number (represented by the symbol Z).

Discovery of the neutron

discovereddiscoverynuclear electrons
All consideration of nuclear electrons ended with James Chadwick's discovery of the neutron in 1932.
By 1920 chemical isotopes had been discovered, the atomic masses had been determined to be (approximately) integer multiples of the mass of the hydrogen atom, and the atomic number had been identified as the charge on the nucleus.

Bohr model

Bohr atomBohr model of the atommodel of the atom
Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in his Bohr model of the atom), decided to test Van den Broek's and Bohr's hypothesis directly, by seeing if spectral lines emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the spectral lines be proportional to the square of Z.
Since this derivation is with the assumption that the nucleus is orbited by one electron, we can generalize this result by letting the nucleus have a charge q = Ze, where Z is the atomic number.

Electron configuration

electronic configurationconfigurationelectronic structure
The configuration of these electrons follows from the principles of quantum mechanics.
Phosphorus (atomic number 15) is as follows: 1s 2 2s 2 2p 6 3s 2 3p 3.

James Chadwick

Sir James ChadwickChadwickChadwick Medal
All consideration of nuclear electrons ended with James Chadwick's discovery of the neutron in 1932.
He looked at the nuclear charge of platinum, silver, and copper, and experimentally found that this was the same as the atomic number within an error of less than 1.5 per cent.

History of the periodic table

Law of Octaveslist of elementsfirst periodic table
The periodic table is an arrangement of the chemical elements, which are organized on the basis of their atomic numbers, electron configurations and recurring chemical properties.