Atomic nucleus: Difference between revisions

[[File:Nucleus drawing.svg|thumb|right|A model of the atomic nucleus showingcollen it as a compact bundle of the two types of [[nucleon]]s: protons (red) and neutrons (blue). In this diagram, protons and neutrons look like little balls stuck together, but an actual nucleus (as understood by modern [[nuclear physics]]) cannot be explained like this, but only by using [[quantum mechanics]]. In a nucleus that occupies a certain [[energy level]] (for example, the [[ground state]]), each nucleon can be said to occupy a range of locations.]]

The '''atomic nucleus'collen'' is the small, dense region consisting of [[proton]]s and [[neutron]]s at the center of an [[atom]], discovered in 1911 by [[Ernest Rutherford]] based on the 1909 [[Geiger–Marsden experiments|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]]<ref>{{cite journal|author=Iwanenko, D.D.|title= The neutron hypothesis|journal= Nature |volume=129|issue= 3265|pages= 798|doi=10.1038/129798d0|year= 1932|bibcode = 1932Natur.129..798I |s2cid= 4096734|doi-access= free}}</ref> and [[Werner Heisenberg]].<ref>{{cite journal |last=Heisenberg |first=W. |title=Über den Bau der Atomkerne. I |journal=[[Z. Phys.]] |volume=77 |issue=1–2 |pages=1–11 |year=1932 |doi=10.1007/BF01342433 |bibcode = 1932ZPhy...77....1H |s2cid=186218053 }}</ref><ref>{{cite journal |last=Heisenberg |first=W. |title=Über den Bau der Atomkerne. II |journal=Z. Phys. |volume=78 |pages=156–164 |year=1932 |doi=10.1007/BF01337585 |issue=3–4 |bibcode = 1932ZPhy...78..156H |s2cid=186221789 }}</ref><ref>{{cite journal |last=Heisenberg |first=W. |title=Über den Bau der Atomkerne. III |journal=Z. Phys. |volume=80 |pages=587–596 |year=1933 |doi=10.1007/BF01335696 |issue=9–10 |bibcode = 1933ZPhy...80..587H |s2cid=126422047 }}</ref><ref>Miller A. I. ''Early Quantum Electrodynamics: A Sourcebook'', Cambridge University Press, Cambridge, 1995, {{ISBN|0521568919}}, pp. 84–88.</ref><ref>{{cite book |author1=Fernandez, Bernard |author2=Ripka, Georges |name-list-style=amp |title=Unravelling the Mystery of the Atomic Nucleus: A Sixty Year Journey 1896 — 1956 |chapter=Nuclear Theory After the Discovery of the Neutron |chapter-url=https://books.google.com/books?id=4PxRBakqFIUC&pg=PA263 |year=2012 |publisher=Springer |isbn=9781461441809 |page=263 }}</ref> An atom is composed of a positively charged nucleus, with a cloud of negatively charged [[electron]]s surrounding it, bound together by [[electrostatic force]]. 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 nucleus was discovered in 1911, as a result of [[Ernest Rutherford]]'s efforts to test Thomson's "[[plum pudding model]]" of the atom.<ref>{{cite web | url =http://www.physics.rutgers.edu/meis/Rutherford.htm | archive-url =https://web.archive.org/web/20011114113713/http://www.physics.rutgers.edu/meis/Rutherford.htm | url-status =dead | archive-date =November 14, 2001 | title =The Rutherford Experiment | publisher =[[Rutgers University]] | access-date =February 26, 2013 }}</ref> The electron had already been discovered by [[J.J. Thomson]]. Knowing that atoms are electrically neutral, J.J.Thomson Thomas 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 the help of [[Ernest Marsden]], that involved the deflection of [[alpha particle]]s (helium nuclei) directed at a thin sheet of metal foil. He reasoned that if J.J 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.

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