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Atomic Nucleus - Wikipedia Atomic nucleus - Wikipedia https://en.wikipedia.org/wiki/Atomic_nucleus#Composition_and_shape Atomic nucleus From Wikipedia, the free encyclopedia The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko[1] and Werner Heisenberg.[2][3] [4][5][6] Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud. Protons and neutrons are bound together to form a nucleus by the nuclear force. The diameter of the nucleus is in the range of 1.75 fm(1.75 × 10−15 m) for hydrogen (the diameter of a single proton)[7] to about 15 fm for the heaviest atoms, such as uranium. These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 23,000 A model of the atomic nucleus showing (uranium) to about 145,000 (hydrogen). it as a compact bundle of the two types of nucleons: protons (red) and neutrons The branch of physics concerned with the study and understanding of the (blue). In this diagram, protons and atomic nucleus, including its composition and the forces which bind it together, neutrons look like little balls stuck is called nuclear physics. together, but an actual nucleus (as understood by modern nuclear physics) cannot be explained like this, but only Contents by using quantum mechanics. In a nucleus which occupies a certain energy level (for example, the ground state), 1 Introduction each nucleon can be said to occupy a 1.1 History range of locations. 1.2 Etymology 1.3 Nuclear makeup 2 Composition and shape 3 Forces 4 Halo nuclei and strong force range limits 5 Nuclear models 5.1 Liquid drop model 5.2 Shell models and other quantum models 5.3 Consistency between models 6 See also 7 References 8 External links Introduction History The nucleus was discovered in 1911, as a result of Ernest Rutherford's efforts to test Thomson's "plum pudding model" of the atom.[8] The electron had already been discovered earlier by J.J. Thomson himself. Knowing that atoms are electrically neutral, Thomson postulated that there must be a positive charge as well. In his plum pudding model, Thomson suggested that an atom consisted of negative electrons randomly scattered within a sphere of positive charge. Ernest Rutherford later devised an experiment with his research partner Hans Geiger and with help of Ernest Marsden, 1 of 7 8/14/2017, 8:27 pm Atomic nucleus - Wikipedia https://en.wikipedia.org/wiki/Atomic_nucleus#Composition_and_shape that involved the deflection of alpha particles (helium nuclei) directed at a thin sheet of metal foil. He reasoned that if Thomson's model were correct, the positively charged alpha particles would easily pass through the foil with very little deviation in their paths, as the foil should act as electrically neutral if the negative and positive charges are so intimately mixed as to make it appear neutral. To his surprise, many of the particles were deflected at very large angles. Because the mass of an alpha particle is about 8000 times that of an electron, it became apparent that a very strong force must be present if it could deflect the massive and fast moving alpha particles. He realized that the plum pudding model could not be accurate and that the deflections of the alpha particles could only be explained if the positive and negative charges were separated from each other and that the mass of the atom was a concentrated point of positive charge. This justified the idea of a nuclear atom with a dense center of positive charge and mass. Etymology The term nucleus is from the Latin word nucleus, a diminutive of nux ("nut"), meaning the kernel (i.e., the "small nut") inside a watery type of fruit (like a peach). In 1844, Michael Faraday used the term to refer to the "central point of an atom". The modern atomic meaning was proposed by Ernest Rutherford in 1912.[9] The adoption of the term "nucleus" to atomic theory, however, was not immediate. In 1916, for example, Gilbert N. Lewis stated, in his famous article The Atom and the Molecule, that "the atom is composed of the kernel and an outer atom or shell"[10] Nuclear makeup The nucleus of an atom consists of neutrons and protons, which in turn are the manifestation of more elementary particles, called quarks, that are held in association by the nuclear strong force in certain stable combinations of hadrons, called baryons. The nuclear strong force extends far enough from each baryon so as to bind the neutrons and protons together against the repulsive electrical force between the positively charged protons. The nuclear strong force has a very short range, and essentially drops to zero just beyond the edge of the nucleus. The collective action of the positively charged nucleus is to hold the electrically negative charged electrons in their orbits about the nucleus. The collection of negatively charged electrons orbiting the nucleus display an affinity for certain configurations and numbers of electrons that make their orbits stable. Which chemical element an atom represents is determined by the number of protons in the nucleus; the neutral atom will have an equal number of electrons orbiting that nucleus. Individual chemical elements can create more stable electron configurations A figurative depiction of the helium-4 atom with the by combining to share their electrons. It is that sharing of electrons electron cloud in shades of gray. In the nucleus, the to create stable electronic orbits about the nucleus that appears to two protons and two neutrons are depicted in red and us as the chemistry of our macro world. blue. This depiction shows the particles as separate, whereas in an actual helium atom, the protons are Protons define the entire charge of a nucleus, and hence its superimposed in space and most likely found at the chemical identity. Neutrons are electrically neutral, but contribute very center of the nucleus, and the same is true of the to the mass of a nucleus to nearly the same extent as the protons. two neutrons. Thus, all four particles are most likely Neutrons explain the phenomenon of isotopes – varieties of the found in exactly the same space, at the central point. same chemical element which differ only in their atomic mass, not Classical images of separate particles fail to model their chemical action. known charge distributions in very small nuclei. A more accurate image is that the spatial distribution of Composition and shape nucleons in a helium nucleus is much closer to the helium electron cloud shown here, although on a far Protons and neutrons are fermions, with different values of the smaller scale, than to the fanciful nucleus image. strong isospin quantum number, so two protons and two neutrons 2 of 7 8/14/2017, 8:27 pm Atomic nucleus - Wikipedia https://en.wikipedia.org/wiki/Atomic_nucleus#Composition_and_shape can share the same space wave function since they are not identical quantum entities. They are sometimes viewed as two different quantum states of the same particle, the nucleon.[11][12] Two fermions, such as two protons, or two neutrons, or a proton + neutron (the deuteron) can exhibit bosonic behavior when they become loosely bound in pairs, which have integral spin. In the rare case of a hypernucleus, a third baryon called a hyperon, containing one or more strange quarks and/or other unusual quark(s), can also share the wave function. However, this type of nucleus is extremely unstable and not found on Earth except in high energy physics experiments. The neutron has a positively charged core of radius ≈ 0.3 fm surrounded by a compensating negative charge of radius between 0.3 fm and 2 fm. The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm.[13] Nuclei can be spherical, rugby ball-shaped (prolate deformation), discus-shaped (oblate deformation), triaxial (a combination of oblate and prolate deformation) or pear-shaped.[14][15] Forces Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is a minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force is much weaker between neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms together internally (for example, the forces that hold the electrons in an inert gas atom bound to its nucleus). The nuclear force is highly attractive at the distance of typical nucleon separation, and this overwhelms the repulsion between protons due to the electromagnetic force, thus allowing nuclei to exist. However, the residual strong force has a limited range because it decays quickly with distance (see Yukawa potential); thus only nuclei smaller than a certain size can be completely stable. The largest known completely stable nucleus (i.e. stable to alpha, beta, and gamma decay) is lead-208 which contains a total of 208 nucleons (126 neutrons and 82 protons). Nuclei larger than this maximum are unstable and tend to be increasingly short-lived with larger numbers of nucleons. However, bismuth-209 is also stable to beta decay and has the longest half-life to alpha decay of any known isotope, estimated at a billion times longer than the age of the universe.
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