Energy levels
   Specific levels in which electrons orbit. They are not really exist, but each electron is orbiting the nucleus in a specific orbit called 'Energy level'. They consist of energy sub-levels, which in turn contains several orbitals.

   Those orbitals have the shape of the electron cloud, which is the region which have the greatest probability of finding electrons. They have many shapes, like S, P, D and F orbitals. Here you can find some of their shapes.

Proton
   Nuclear particle having a positive charge identical in magnitude to the negative charge of an electron and, together with the neutron, a constituent of all atomic nuclei. The proton is also called a nucleon, as is the neutron. A single proton forms the nucleus of the hydrogen atom. The mass of a proton is 1.6726 × 10^-27 kg, or approximately 1,836 times that of an electron.

   Consequently, the mass of an atom is contained almost entirely in the nucleus. The proton has an intrinsic angular momentum, or spin, and thus a magnetic moment. In addition, the proton obeys the exclusion principle. The atomic number of an element denotes the number of protons in the nucleus and determines what element it is. In nuclear physics the proton is used as a projectile in large accelerators to bombard nuclei to produce fundamental particles. As the hydrogen ion, the proton plays an important role in chemistry.

   The antiproton, the antiparticle of the proton, is also called a negative proton. It differs from the proton in having a negative charge and not being a constituent of atomic nuclei. The antiproton is stable in a vacuum and does not decay spontaneously. When an antiproton collides with a proton or a neutron, however, the two particles are transformed into mesons, which have an extremely short half-life. Although physicists first postulated the existence of this elementary particle in the 1930s, the antiproton was positively identified for the first time in 1955 at the University of California Radiation Laboratory.

   Protons are essential parts of ordinary matter and are stable over periods of billions and even trillions of years. Particle physicists are nevertheless interested in learning whether protons eventually decay, on a timescale of 1033 years or more. This interest derives from current attempts at grand unification theories that would combine all four fundamental interactions of matter in a single scheme. Many of these attempts entail the ultimate instability of the proton, so research groups at a number of accelerator facilities are conducting tests to detect such decays. No clear evidence has yet been found; possible indications thus far can be interpreted in other ways.

Neutron
   Uncharged particle, one of the fundamental particles of which matter is composed. The mass of a neutron is 1.675 × 10^-27 kg, about one eighth of one per cent heavier than the proton. The existence of the neutron was predicted in 1920 by the British physicist Ernest Rutherford and by Australian and American scientists, but experimental verification of its existence was difficult because the net electrical charge on the neutron is zero. Most particle detectors register charged particles only.

Discovery
   The neutron was first identified in 1932 by the British physicist James Chadwick, who correctly interpreted the results of experiments conducted at that time by the French physicists Irène and Frédéric Joliot-Curie and other scientists. The Joliot-Curies had produced a previously unknown kind of radiation by the interaction of alpha particles with beryllium nuclei. When this radiation was passed through paraffin wax, collisions between the neutrons and the hydrogen atoms in the wax produced readily detectable protons. Chadwick recognized that the radiation consisted of neutrons.

Behavior 
   The neutron is a constituent particle of all nuclei of mass number greater than 1; that is, of all nuclei except ordinary hydrogen. Free neutrons-those outside atomic nuclei-are produced in nuclear reactions. They can be ejected from atomic nuclei at various speeds or energies and are readily slowed down to very low energy by a series of collisions with light nuclei, such as those of hydrogen, deuterium, or carbon. When expelled from the nucleus, the neutron is unstable and decays to form a proton, an electron, and a neutrino. Like the proton and the electron, the neutron possesses angular momentum, or spin. Neutrons act as small, individual magnets; this property enables beams of polarized neutrons to be created. The neutron has a negative magnetic moment of -1.913141 nuclear magnetons or approximately a thousandth of a Bohr magneton. The currently accepted value of its half-life is 615 s +/- 1.4 s. The corresponding value of the mean life, which is now more commonly used, is 887 s +/- 2s.

   The antiparticle of a neutron, known as an antineutron, has the same mass, spin, and rate of beta decay. These particles are sometimes produced in the collisions of antiprotons with protons, and they possess a magnetic moment equal and opposite to that of the neutron. According to current particle theory, the neutron and the antineutron-and other nuclear particles-are themselves composed of quarks.

Neutron Radiography 
   An increasingly important application of reactor-generated neutrons is neutron radiography, in which information is obtained by determining the absorption of a beam of neutrons emanating from a nuclear reactor or a powerful radioisotope source. The technique resembles X-ray radiography. Many substances, however, such as metals that are opaque to X-rays, will transmit neutrons; other substances (particularly hydrogen compounds) that transmit X-rays are opaque to neutrons. A neutron radiograph is made by exposing a thin foil to a beam of neutrons that has penetrated the test object. The neutrons leave an invisible radioactive "picture" of the object on the foil. A visible picture is made by placing a photographic film in contact with the foil. A direct, television-like technique for viewing images has also been developed.

   First used in Europe in the 1930s, neutron radiography has been employed widely since the 1950s for examining nuclear fuel and other components of reactors. More recently it has been used in examining explosive devices and components of space vehicles. Beams of neutrons are widely used now in the physical and biological sciences and in technology, and neutron activation analysis is an important tool in such diverse fields as paleontology, archaeology, and art history.

Electron
   A type of elementary particle that, along with protons and neutrons, makes up atoms and molecules. Electrons play a role in a wide variety of phenomena. The flow of an electric current in a metallic conductor is caused by the drifting of free electrons in the conductor. Heat conduction in a metal is also primarily a phenomenon of electron activity. In vacuum tubes a heated cathode emits a stream of electrons that can be used to amplify or rectify an electric current If such a stream is focused into a well-defined beam, it is called a cathode-ray beam. Cathode rays directed against suitable targets produce X-rays; directed against the fluorescent screen of a television tube, they produce visible images. The negatively charged beta particles emitted by some radioactive substances are electrons.

   Electrons have a rest mass of 9.109 x 10^-31 kg and a negative electrical charge of 1.602 x 10^-19 coulombs. Electrons are classified as fermions because they have half-integral spin; spin is a quantum-mechanical property of subatomic particles that indicates the particle's angular momentum. The antimatter counterpart of the electron is the positron.

Positron
   Elementary antimatter particle having a mass equal to that of an electron and a positive electrical charge equal in magnitude to the charge of the electron. The positron is sometimes called a positive electron or anti-electron. Electron-positron pairs can be formed if gamma rays with energies of more than 1 million electron volts strike particles of matter. The reverse of the pair-production process, called annihilation, occurs when an electron and a positron interact, destroying each other and producing gamma rays.

   The existence of the positron was first suggested in 1928 by the British physicist P. A. M. Dirac as a necessary consequence of his quantum-mechanical theory of electron motion. In 1932 the American physicist Carl Anderson confirmed the existence of the positron experimentally.

Alpha Particle
   Positively charged nuclear particle, symbol a, consisting of two protons bound to two neutrons. Alpha particles are emitted spontaneously in some types of radioactive decay. They consist of completely ionized helium-4 atoms

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