Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)
Induced fission reaction. A neutron is absorbed by a uranium-235 nucleus, turning it briefly into an excited uranium-236 nucleus, with the excitation energy provided by the kinetic energy of the neutron plus the forces that bind the neutron. The uranium-236, in turn, splits into fast-moving lighter elements (fission products) and releases several free neutrons, one or more "prompt gamma rays" (not shown) and a (proportionally) large amount of energy.
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.
A visual representation of an induced nuclear fission event where a slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which fissions into two fast-moving lighter elements (fission products) and additional neutrons. Most of the energy released is in the form of the kinetic velocities of the fission products and the neutrons.
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 This model of electrons in orbits is obsolete.
Fission product yields by mass for thermal neutron fission of U-235, Pu-239, a combination of the two typical of current nuclear power reactors, and U-233 used in the thorium cycle.
The binding energy needed for a nucleon to escape the nucleus, for various isotopes
The stages of binary fission in a liquid drop model. Energy input deforms the nucleus into a fat "cigar" shape, then a "peanut" shape, followed by binary fission as the two lobes exceed the short-range nuclear force attraction distance, then are pushed apart and away by their electrical charge. In the liquid drop model, the two fission fragments are predicted to be the same size. The nuclear shell model allows for them to differ in size, as usually experimentally observed.
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.
Animation of a Coulomb explosion in the case of a cluster of positively charged nuclei, akin to a cluster of fission fragments. Hue level of color
is proportional to (larger) nuclei charge. Electrons (smaller) on this time-scale are seen only stroboscopically and the hue level is their kinetic energy
3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher are not shown)
The "curve of binding energy": A graph of binding energy per nucleon of common isotopes.
This diagram shows the half-life (T½) of various isotopes with Z protons and N neutrons.
A schematic nuclear fission chain reaction. 1. A uranium-235 atom absorbs a neutron and fissions into two new atoms (fission fragments), releasing three new neutrons and some binding energy. 2. One of those neutrons is absorbed by an atom of uranium-238 and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. However, the one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. 3. Both of those neutrons collide with uranium-235 atoms, each of which fissions and releases between one and three neutrons, which can then continue the reaction.
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.
The cooling towers of the Philippsburg Nuclear Power Plant, in Germany.
An example of absorption lines in a spectrum
The mushroom cloud of the atomic bomb dropped on Nagasaki, Japan, on 9 August 1945 rose over 18 km above the bomb's hypocenter. An estimated 39,000 people were killed by the atomic bomb, of whom 23,145–28,113 were Japanese factory workers, 2,000 were Korean slave laborers, and 150 were Japanese combatants.
Graphic illustrating the formation of a Bose–Einstein condensate
Hahn and Meitner in 1912
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).
Experimental apparatus similar to that with which Otto Hahn and Fritz Strassmann discovered nuclear fission in 1938. The apparatus would not have been on the same table or in the same room.
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.
Drawing of the first artificial reactor, Chicago Pile-1.

Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei.

- Nuclear fission

In this case, the nucleus splits and leaves behind different elements.

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

9 related topics

Alpha

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

Neutron

Subatomic particle, symbol or, which has a neutral charge, and a mass slightly greater than that of a proton.

Subatomic particle, symbol or, which has a neutral charge, and a mass slightly greater than that of a proton.

The quark content of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.
Nuclear fission caused by absorption of a neutron by uranium-235. The heavy nuclide fragments into lighter components and additional neutrons.
Models depicting the nucleus and electron energy levels in hydrogen, helium, lithium, and neon atoms. In reality, the diameter of the nucleus is about 100,000 times smaller than the diameter of the atom.
A schematic of the nucleus of an atom indicating radiation, the emission of a fast electron from the nucleus (the accompanying antineutrino is omitted). In the Rutherford model for the nucleus, red spheres were protons with positive charge and blue spheres were protons tightly bound to an electron with no net charge. 
The inset shows beta decay of a free neutron as it is understood today; an electron and antineutrino are created in this process.
The Feynman diagram for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W boson
The leading-order Feynman diagram for decay of a proton into a neutron, positron, and electron neutrino via an intermediate boson.
Institut Laue–Langevin (ILL) in Grenoble, France – a major neutron research facility.
Cold neutron source providing neutrons at about the temperature of liquid hydrogen
The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The D–T rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.
Transmutation flow in light water reactor, which is a thermal-spectrum reactor

Protons and neutrons constitute the nuclei of atoms.

Neutrons are produced copiously in nuclear fission and fusion.

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Nuclear binding energy

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A graphical representation of the semi-empirical binding energy formula. The binding energy per nucleon in MeV (highest numbers in yellow, in excess of 8.5 MeV per nucleon) is plotted for various nuclides as a function of Z, the atomic number (y-axis), vs. N, the number of neutrons (x-axis). The highest numbers are seen for Z = 26 (iron).

Nuclear binding energy in experimental physics is the minimum energy that is required to disassemble the nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons.

If new binding energy is available when light nuclei fuse (nuclear fusion), or when heavy nuclei split (nuclear fission), either process can result in release of this binding energy.

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.

Three other radioactive elements, technetium, promethium, and neptunium, occur only incidentally in natural materials, produced as individual atoms by nuclear fission of the nuclei of various heavy elements or in other rare nuclear processes.

The nucleus of a helium atom. The two protons have the same charge, but still stay together due to the residual nuclear force

Strong interaction

Fundamental interaction that confines quarks into proton, neutron, and other hadron particles.

Fundamental interaction that confines quarks into proton, neutron, and other hadron particles.

The nucleus of a helium atom. The two protons have the same charge, but still stay together due to the residual nuclear force
The fundamental couplings of the strong interaction, from left to right: gluon radiation, gluon splitting and gluon self-coupling.
An animation of the nuclear force (or residual strong force) interaction between a proton and a neutron. The small colored double circles are gluons, which can be seen binding the proton and neutron together. These gluons also hold the quark/antiquark combination called the pion together, and thus help transmit a residual part of the strong force even between colorless hadrons. Anticolors are shown as per [[:File:Quark Anticolors.svg|this diagram]]. For a larger version, click here

On a larger scale (of about 1 to 3 femtometer), it is the force (carried by mesons) that binds protons and neutrons (nucleons) together to form the nucleus of an atom.

Additionally, distinctions exist in the binding energies of the nuclear force of nuclear fusion vs nuclear fission.

Heisenberg in 1933

Werner Heisenberg

German theoretical physicist and one of the key pioneers of quantum mechanics.

German theoretical physicist and one of the key pioneers of quantum mechanics.

Heisenberg in 1933
Heisenberg in 1924
A visual representation of an induced nuclear fission event where a slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which fissions into two fast-moving lighter elements (fission products) and additional neutrons. Most of the energy released is in the form of the kinetic velocities of the fission products and the neutrons.
Replica of the German experimental nuclear reactor captured and dismantled at Haigerloch
Bust of Heisenberg in his old age, on display at the Max Planck Society campus in Garching bei München

He recounted philosophical conversations with his fellow students and teachers about understanding the atom while receiving his scientific training in Munich, Göttingen and Copenhagen.

Meitner, and her nephew Otto Robert Frisch, correctly interpreted Hahn's and Strassmann's results as being nuclear fission.

Deuterium

One of two stable isotopes of hydrogen (the other being protium, or hydrogen-1).

One of two stable isotopes of hydrogen (the other being protium, or hydrogen-1).

Deuterium discharge tube
Ionized deuterium in a fusor reactor giving off its characteristic pinkish-red glow
Emission spectrum of an ultraviolet deuterium arc lamp
Harold Urey, deuterium's discoverer
The "Sausage" device casing of the Ivy Mike H bomb, attached to instrumentation and cryogenic equipment. The 20-ft-tall bomb held a cryogenic Dewar flask with room for 160 kg of liquid deuterium.

The nucleus of a deuterium atom, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutrons in the nucleus.

In this context, it was the first bomb in which most of the energy released came from nuclear reaction stages that followed the primary nuclear fission stage of the atomic bomb.

Bohr in 1922

Niels Bohr

Bohr in 1922
Bohr as a young man
Bohr and Margrethe Nørlund on their engagement in 1910.
The Bohr model of the hydrogen atom. A negatively charged electron, confined to an atomic orbital, orbits a small, positively charged nucleus; a quantum jump between orbits is accompanied by an emitted or absorbed amount of electromagnetic radiation.
The evolution of atomic models in the 20th century: Thomson, Rutherford, Bohr, Heisenberg/Schrödinger
The Niels Bohr Institute, part of the University of Copenhagen
Bohr and Albert Einstein (image from 1925) had a long-running debate about the metaphysical implication of quantum physics.
Werner Heisenberg (left) with Bohr at the Copenhagen Conference in 1934
Bohr with James Franck, Albert Einstein and Isidor Isaac Rabi (LR)
Bohr's coat of arms, 1947. Argent, a taijitu (yin-yang symbol) Gules and Sable. Motto: Contraria sunt complementa ("opposites are complementary").
The Theory of Spectra and Atomic Constitution (Drei Aufsätze über Spektren und Atombau), 1922

Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922.

The discovery of nuclear fission by Otto Hahn in December 1938 (and its theoretical explanation by Lise Meitner) generated intense interest among physicists.

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.

This corresponds to the fact that nuclear fission in an element heavier than zirconium produces energy, and fission in any element lighter than niobium requires energy.

A physicist observes alpha particles from the decay of a polonium source in a cloud chamber

Alpha particle

Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus.

Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus.

A physicist observes alpha particles from the decay of a polonium source in a cloud chamber
Alpha radiation detected in an isopropanol cloud chamber (after injection of an artificial source radon-220).
Example selection of radioactive nuclides with main emitted alpha particle energies plotted against their atomic number. Note that each nuclide has a distinct alpha spectrum.
Alpha radiation consists of helium-4 nucleus and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons, is halted by an aluminium plate. Gamma radiation is eventually absorbed as it penetrates a dense material. Lead is good at absorbing gamma radiation, due to its density.
An alpha particle is deflected by a magnetic field
Dispersing of alpha particles on a thin metal sheet
Energy-loss (Bragg curve) in air for typical alpha particle emitted through radioactive decay.
The trace of a single alpha particle obtained by nuclear physicist Wolfhart Willimczik with his spark chamber specially made for alpha particles.

A higher energy version of alphas than produced in alpha decay is a common product of an uncommon nuclear fission result called ternary fission.

When an atom emits an alpha particle in alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle.