Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)
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 Geiger–Marsden experiment:
Left: Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection.
Right: Observed results: a small portion of the particles were deflected by the concentrated positive charge of the nucleus.
The Rutherford–Bohr model of the hydrogen atom or a hydrogen-like ion (Z > 1). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one orbital to another be proportional to the mathematical square of atomic charge (Z2). Experimental measurement by Henry Moseley of this radiation for many elements (from ) showed the results as predicted by Bohr. Both the concept of atomic number and the Bohr model were thereby given scientific credence.
The Bohr model of the atom, with an electron making instantaneous "quantum leaps" from one orbit to another with gain or loss of energy. This model of electrons in orbits is obsolete.
Russian chemist Dmitri Mendeleev, creator of the periodic table.
The binding energy needed for a nucleon to escape the nucleus, for various isotopes
Niels Bohr, creator of the Bohr model.
A potential well, showing, according to classical mechanics, the minimum energy V(x) needed to reach each position x. Classically, a particle with energy E is constrained to a range of positions between x1 and x2.
Henry Moseley in his lab.
3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher are not shown)
This diagram shows the half-life (T½) of various isotopes with Z protons and N neutrons.
These electron's energy levels (not to scale) are sufficient for ground states of atoms up to cadmium (5s2 4d10) inclusively. Do not forget that even the top of the diagram is lower than an unbound electron state.
An example of absorption lines in a spectrum
Graphic illustrating the formation of a Bose–Einstein condensate
Scanning tunneling microscope image showing the individual atoms making up this gold (100) surface. The surface atoms deviate from the bulk crystal structure and arrange in columns several atoms wide with pits between them (See surface reconstruction).
Periodic table showing the origin of each element. Elements from carbon up to sulfur may be made in small stars by the alpha process. Elements beyond iron are made in large stars with slow neutron capture (s-process). Elements heavier than iron may be made in neutron star mergers or supernovae after the r-process.

For ordinary nuclei, this is equal to the proton number (np) or the number of protons found in the nucleus for every atom of that element.

- Atomic number

The number of protons in the nucleus is the atomic number and it defines to which chemical element the atom belongs.

- Atom
Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)

8 related topics

Alpha

The chemical elements ordered in the periodic table

Chemical element

The chemical elements ordered in the periodic table
Estimated distribution of dark matter and dark energy in the universe. Only the fraction of the mass and energy in the universe labeled "atoms" is composed of chemical elements.
Periodic table showing the cosmogenic origin of each element in the Big Bang, or in large or small stars. Small stars can produce certain elements up to sulfur, by the alpha process. Supernovae are needed to produce "heavy" elements (those beyond iron and nickel) rapidly by neutron buildup, in the r-process. Certain large stars slowly produce other elements heavier than iron, in the s-process; these may then be blown into space in the off-gassing of planetary nebulae
Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers (the Oddo-Harkins rule), and (2) a general decrease in abundance as elements become heavier. Iron is especially common because it represents the minimum energy nuclide that can be made by fusion of helium in supernovae.
Mendeleev's 1869 periodic table: An experiment on a system of elements. Based on their atomic weights and chemical similarities.
Dmitri Mendeleev
Henry Moseley

A chemical element refers to all aspects of the species of atoms that have a certain number of protons in their nuclei, including the pure substance consisting only of that species.

The number of protons in the nucleus is the defining property of an element, and is referred to as its atomic number (represented by the symbol Z) – all atoms with the same atomic number are atoms of the same element.

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Periodic table

Tabular display of the chemical elements.

Tabular display of the chemical elements.

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3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher are not shown)
Idealized order of shell-filling (most accurate for n  ≲ 4.)
Trend in atomic radii
Graph of first ionisation energies of the elements in electronvolts (predictions used for elements 105–118)
Trend in electron affinities
Flowing liquid mercury. Its liquid state at room temperature is a result of special relativity.
A periodic table colour-coded to show some commonly used sets of similar elements. The categories and their boundaries differ somewhat between sources. Alkali metals
 Alkaline earth metals
 Lanthanides
 Actinides
 Transition metals Other metals
 Metalloids
 Other nonmetals
 Halogens
 Noble gases
Mendeleev's 1869 periodic table
Mendeleev's 1871 periodic table
Dmitri Mendeleev
Henry Moseley
Periodic table of van den Broek
Glenn T. Seaborg
One possible form of the extended periodic table to element 172, suggested by Finnish chemist Pekka Pyykkö. Deviations from the Madelung order (8s < < 6f < 7d < 8p) begin to appear at elements 139 and 140, though for the most part it continues to hold approximately.
Otto Theodor Benfey's spiral periodic table (1964)
Iron, a metal
Sulfur, a nonmetal
Arsenic, an element often called a semi-metal or metalloid

It is a graphic formulation of the periodic law, which states that the properties of the chemical elements exhibit a periodic dependence on their atomic numbers.

The smallest constituents of all normal matter are known as atoms.

The quark content of a proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.

Proton

Stable subatomic particle, symbol, H+, or 1H+ with a positive electric charge of +1e elementary charge.

Stable subatomic particle, symbol, H+, or 1H+ with a positive electric charge of +1e elementary charge.

The quark content of a proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.
Ernest Rutherford at the first Solvay Conference, 1911
Proton detected in an isopropanol cloud chamber
Protium, the most common isotope of hydrogen, consists of one proton and one electron (it has no neutrons). The term "hydrogen ion" implies that that H-atom has lost its one electron, causing only a proton to remain. Thus, in chemistry, the terms "proton" and "hydrogen ion" (for the protium isotope) are used synonymously

One or more protons are present in the nucleus of every atom.

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).

Moseley in 1914

Henry Moseley

Moseley in 1914
Blue plaque erected by the Royal Society of Chemistry on the Townsend Building of Oxford's Clarendon Laboratory, commemorating Moseley's work on X-rays emitted by elements

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.

That theory refined Ernest Rutherford's and Antonius van den Broek's model, which proposed that the atom contains in its nucleus a number of positive nuclear charges that is equal to its (atomic) number in the periodic table.

The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.

Hydrogen

The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the Bohr model radius (image not to scale)
Hydrogen gas is colorless and transparent, here contained in a glass ampoule.
Phase diagram of hydrogen. The temperature and pressure scales are logarithmic, so one unit corresponds to a 10x change. The left edge corresponds to 105 Pa, which is about atmospheric pressure.
A sample of sodium hydride
Hydrogen discharge (spectrum) tube
Deuterium discharge (spectrum) tube
Antoine-Laurent de Lavoisier
Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the Balmer series
NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy
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Hydrogen is the chemical element with the symbol H and atomic number 1.

For the most common isotope of hydrogen (symbol 1H) each atom has one proton, one electron, and no neutrons.

The quark content of a proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.

Mass number

Total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

Total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

The quark content of a proton. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.

It is approximately equal to the atomic (also known as isotopic) mass of the atom expressed in atomic mass units.

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.

Electron atomic and molecular orbitals

Electron configuration

Electron atomic and molecular orbitals
A Bohr diagram of lithium
The approximate order of filling of atomic orbitals, following the arrows from 1s to 7p. (After 7p the order includes subshells outside the range of the diagram, starting with 8s.)
Electron configuration table showing blocks.

In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals.

Phosphorus (atomic number 15) is as follows: 1s2 2s2 2p6 3s2 3p3.

Stylized lithium-7 atom: 3 protons, 4 neutrons, and 3 electrons (total electrons are ~1⁄4300th of the mass of the nucleus). It has a mass of 7.016 Da. Rare lithium-6 (mass of 6.015 Da) has only 3 neutrons, reducing the atomic weight (average) of lithium to 6.941.

Atomic mass

Stylized lithium-7 atom: 3 protons, 4 neutrons, and 3 electrons (total electrons are ~1⁄4300th of the mass of the nucleus). It has a mass of 7.016 Da. Rare lithium-6 (mass of 6.015 Da) has only 3 neutrons, reducing the atomic weight (average) of lithium to 6.941.
Binding energy per nucleon of common isotopes. A graph of the ratio of mass number to atomic mass would be similar.

The atomic mass (ma or m) is the mass of an atom.

Additionally, the neutron count (neutron number) may then be derived by subtracting the number of protons (atomic number) from the mass number (nucleon count).