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The nucleus (atomic nucleus) is the center of an atom. It is composed of one or more protons and usually some neutrons as well. The number of protons in an atom's nucleus is called the atomic number, and determines which element the atom is (for example hydrogen, carbon, oxygen, etc.). The number of neutrons determines the isotope of the element. The numbers of protons and neutrons in a nucleus are correlated; generally they are approximately equivalent. Protons and neutrons have nearly equal masses, and their combined number determines the atomic mass of an atom (each isotope of an element has a unique atomic mass). Electrons have negligible mass when compared to the nucleus and do not contribute significantly to atomic mass.
Though the positively charged protons exert a repulsive electromagnetic force on each other, the distances between nuclear particles are small enough that the strong interaction (which is stronger than the electromagnetic force but decreases more rapidly with distance) predominates. (The gravitational attraction is negligible, being a factor 1036 weaker than this electromagnetic repulsion.)
The discovery of the electron was the first indication that the atom had internal structure. This structure was initially imagined according to the "raisin cookie" or "plum pudding" modelThe Plum pudding model of the atom was made after the discovery of the electron but before the discovery of the proton or neutron. In it, the atom is envisioned as electrons surrounded by a soup of positive charge, like plums surrounded by pudding. This m, in which the small, negatively charged electrons were embedded in a large sphere containing all the positive charge. Ernest Rutherford and Marsden, however, discoveredRutherford scattering is a phenomenon that was observed by Ernest Rutherford in 1911 that led to the development of the orbital theory of the atom. It is now exploited by the materials analytical technique Rutherford backscattering. Rutherford scattering in their famous 1911 gold foil experimentThe Gold foil experiment was an experiment done by Ernest Rutherford to determine the layout of the atom. Until that time, the prevailing theory was the Plum pudding model of the atom. Rutherford determined that the true shape is, in fact, the Bohr model that alpha particleAlpha particles or alpha rays are a form of particle radiation which are highly ionizing and have low penetration. They consist of two protons and two neutrons bound together into a particle that is identical to a helium nucleus, and can be written as He2s from a radium source were sometimes scattered backwards from a gold foil, which led to the acceptance of the Bohr modelThe Bohr Model is a physical model that depicts the atom as a small positively charged nucleus with electrons in orbit at different levels, similar in structure to the solar system. It is named for the Danish physicist Niels Bohr. Developments in the fiel, a planetary model in which the electrons orbited a tiny nucleus in the same way that the planets orbit the sun.
A heavy nucleus can contain hundreds of nucleonNucleon is the common name used in nuclear chemistry to refer to a neutron or a proton, the components of an atom's nucleus. The total number of nucleons in an atom is the mass number on the atom, as nucleons each have a mass of one amu. See also List ofs (neutrons and protons), which means that to some approximation it can be treated as a classical system, rather than a quantum-mechanical one. In the resulting liquid-drop model, the nucleus has an energy which arises partly from surface tension and partly from electrical repulsion of the protons. The liquid-drop model is able to reproduce many features of nuclei, including the general trend of binding energy with respect to mass number, as well as the phenomenon of nuclear fission.
Superimposed on this classical picture, however, are quantum-mechanical effects, which can be described using the nuclear shell model, developed in large part by Maria Goeppert-Mayer . Nuclei with certain numbers of neutrons and protons (the magic numbers 2, 8, 20, 50, 82, 126, ...) are particularly stable, because their shells are filled.
Since some nuclei are more stable than others, it follows that energy can be released by nuclear reactions. The sun is powered by nuclear fusion, in which two nuclei collide and merge to form a larger nucleus. The opposite process is fission, which powers nuclear power plants. Because the binding energy per nucleon is at a maximum for medium-mass nuclei (around iron), energy is released either by fusing light nuclei or by fissioning heavier ones.
The elements up to iron are created in a star during a series of fusion stages. First hydrogen fuses with itself to form helium, then helium fuses with itself twice to make carbon, and further fusings proceed to make heavier elements, until the series of fusions make iron which will not fuse further. If the star explodes in a supernova, the high energy neutrinos streaming from the supernova will bombard the escaping elements to form substantial portions of the elemental neuclei heavier than iron. Hence, during stellar evolution through the progression of stages in fusing succeedingly heavier elements, the death of a star in a supernova can create the elements necessary for life.
Nuclear reactions occur naturally on earth. Except in manmade conditions, such as atomic explosions, temperatures and pressures on earth are not high enough to overcome the electrical repulsion between nuclei and allow fusion. But heavy nuclei such as uranium may undergo fission and alpha decay, and beta decay can also occur. Alpha decay can be considered as an extremely asymmetric case of fission, in which one fragment is a helium nucleus ( alpha particle). In beta decay, either a proton is converted into a neutron (with the emission of an antielectron and a neutrino) or a neutron is converted into a proton (emitting an electron and an antineutrino).
Much of current research in nuclear physics relates to the study of nuclei under extreme conditions. The heaviest of all nuclei are neutron stars. Nuclei may also be characterized by extreme shapes (like footballs) or by extreme neutron-to-proton ratios. Experimenters can also use artificially induced fusion at high energies to create nuclei at very high temperatures, and there are signs that these experiments have produced a phase transition from normal nuclear matter to a new state, the quark-gluon plasma, in which the quarks mingle with one another, rather than being segregated in triplets as neutrons and protons.
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