History of Atomic Theory

History of Atomic TheoryThe hintic possible action is developed since 2000 years ago the Greek philosopher Democritus proposed that there was a limit to how sm every one could be divide matter, this smallest matter indivisible particle was called mote. However this atomic theory of Democritus was criticized by Aristotle who proposed a influence based on four basic elements of earth, air, fire and water.Aristotles view held for the next 2000 years as it better suited ghostlike beliefs of the time. In 1801, an English teacher named John Dalton proposed his atomic theory which stated matter is composed of all small indivisible atoms, elements contain one font of atom different elements contain different atoms, compounds contain more than one type of atom. In 1904, British physicist J.J Thomson and others demonstrated that cathode rays (electrons) were present in all matter.Thomson proposed that the atom was a sphere of positive charge in which embedded were rings of negative charg es (electrons) , like plums in a pudding. Lather on, in 19 century, the discoveries of radiotherapy leads to a new progress of Physics. We knew that certain elements emitting radiation, this suggested that atoms be no longer indivisible and non indestructible, as proposed in Daltons atomic simulation.Radiation provides an valuable gumshoe for the study of matter. In 1911, Rutherford first suggested the use of alpha particles to probe the internal structure of the atom. Finally, the nucleus and its protons were discovered.Rutherfords scattering experimentErnest Rutherford, directed an experiment to Hans Geiger and Ernest Marsden in 1909, in which the newly discovered alpha particles (Helium nuclei) were fired at a thin gold foil layer which only a few atom thick. At that time the atom was thought to be analogous plum pudding by Thomson with a negative charge (the plum) throughout the positive sphere (the pudding). just about of the alpha particle passed through with no or only very small deflections in a vacuum (see figure), as would been excepted on the Thomson seat of the atom current at the time. About 1 in 8000 was deflected through angles greater than 90 degree. The result was so unexpected that Rutherford was very unexpected that Rutherford was promoted to write.it was near incredible as if you fired a 15 inch shell at a piece of tissue and it came back and hit you.Therefore Rutherford reason that majority of the mass of the atom was concentrated in the nucleus. The small coat of the nucleus explained the small number of the alpha particles that were repelled each other in this way. Rutherford showed that the size of the nucleus was less than about 10 -14m.Bohrs model, how it fit experimental observationAccelerated electrons emitted and lose verve which predicted by Predicted by Maxwell and confirmed by Hertz, however the electron doesnt spin into the nucleus because of vigor losing.Later on, In 1903.H.G.J Morseley found simple, regular relatio nship between the frequencies of X-ray emission crimp, thus it provided an manifest to support Bohrs model. Scientist began to work on an alternating model to replace it.Bohrs postulatesIn 1913 the Danish physicist Niels Bohr (18851992), put forward some radical propositions to account for the discrepancies between Rutherfords model of the atom and the available experimental evidence. Bohrs postulates are1. Electron can remove in certain allowed orbitsstationary states ( vitality) without radiating energy.2.when an electron falls from a higher energy level to a lower energy level, it emits energy that is quantised by the plank relationship E2 E 1= hf.3. Augular momentum (mvr) is quantised and can only take determine of the nh / 2 when n is the principal quantum number.The first postulate account for the stability of the atom. However why the these stationary state excited was unknown. They exist was a fact.The arcminute postulate explains the line emission spectra. Emission (or absorption) of Energy is discontinuous and corresponds to a transition between two stationary states. Since the energy can be quantized, the emission, the oftenness of the emitted (or passed) radiation is predetermined. A transition between different states will lead to difference frequencies or colours.The third postulate effectively sets limits on the radius of the allowed orbits.Bohrs modelBohr realised that if his model was correct, each atom would have a apparitional fingerprint to the differences between electron energy level in that atom. The Rydberg equating which also known as Balmer equation, has given him evidence for the quantised emission of the energy from hydrogen atom, leading him to going on to further his model and define his postulate.So the hydrogen spectrum was very significant to the development of Bohrs model of the atom, because without the netherstanding of it, Bohr may not continue his work of the model.Produced and observable when hydrogen gas was excite d by the addition of energy. The equation in the original form was modified by Rhydberg until it worked and could be applied to explain the spectrum of hydrogen by using integer determine of n, only as suggested by Bohr in his postulates.Quantum number and quantum changesIts possible to determine the energy of each orbit using Bohrs model and from this construct an energy level Figure for hydrogen. The figure below shows the energy these energy levels. Alternatively a transition between stationary states can be show in figure. B. The Balmer series of lines occurs when the electrons fall to the n=2 level from n=2 level n=3,4,5 and 6 levels. This is illustrated differently in figure. B.How Bohr describes the hydrogen spectraBohrs model of the atom cluster quite similar to that of Rutherfords with two important differences firstly, it assisted positions to the electrons, but secondly the electron energy level s were quantised.This was radically new, the idea that electrons had energ y states and could absorb and emit energy to change states, and had no evidence. Bohr realised that if his model was correct, each atom would have a spectral fingerprint to the differences between electron energy level in that atom.The Rydberg equation provided him evidence for the quantised emission of the energy from hydrogen atom. It leads him to going on to further his model and defines his postulate. So the hydrogen spectrum was important to the development of Bohrs model of the atom.The energy levels describe by Bohr is clearly marked. According to Bohr, the Balmer series (shown on the bottom of the plot as the hydrogen spectrum) was cause by changing energy levels, in the process releasing light. As shown, larger energy changes produce more energetic photons, as seen in Balmers series, as further, this diagram shows how the Balmer series is formed by successive electron transition to the 2ndshell (transition to other shell produce additional lines named after their discoveri es.)This is a great achievement that the Bohr s model is able to provide a physical basis for the Balmer series formula. From his second postulate E f Ei= hf. (i) states for initial energy level (f)states for terminal energy levelEi=1/ni2E1and Ef=1/nf2E1hence hf =1/ nf2E1-1/ ni2E1=(1/ nf2-1/ ni2) E1And since c=fA=A=c/fthe expression reduces to 1/A=E1/hcX(1/ nf2-1/ ni2)where R states for Rydbergs constant, RH(hydrogen)1.097X 107m-1.By using the mixture of classical physics and quantum physics, Bohr was able to define the equation for the spectral lines of hydrogen. He didnt know why the electrons obeyed his rules. These were purely empirical results.Problems with the modelFor all the success, the Bohr model of the atom had serious limitations It and ad hoc mixture of classical and quantum physics it allows some laws of the classical physics held and others did not. Hydrogen has only one electron, and Bohrs postulate are only able to explain it. It cant work fo r multi-electron atoms .It could not explain the relative intensities of the spectral line some lines were more fierce to the others and it was not known why this should occur. Certain spectral line were foun of a number of very fine and close lines and the cause of these hyperfine spectral lines could not be explained. The splitting of spectral lines when the sample was placed in a magnetic field (called the Zeeman effect, and discussed below) could also not be explained.The postulates faced a line of work that it is suited for larger atom. Hydrogen is the simplest atom containing only one electron. Similarity He+and Li+have one electron. Bohrs model works with theses atom and ions. In all the other atoms however the electron interact with each other. In a larger atoms the outer electrons are shielded from the nucleus by the interior electrons. Interaction between electrons also result in different energy levels. The affect Bohrs model to the extent that the spectra of multiple electrons could not be explain.When the spectrum of the hydrogen was examined it was remark that the emission line varied in intensity. Some were quite intense and others were less intense some were sharp and some were boarder. The following figure. illustrate these differences. Bohrs model could not explain these features but later it was explained that electron orbited in a ellipse and not in a circles. As the developing of the light spectroscopes ameliorate it was found that some of the spectral lines were made up with hyperfine lines. This suggestion spitted Bohrs energy level theory however there was no explanation for this.The Zeeman perfumeZeeman Effect occurred when a magnetic field us pass through a discharge tube. The magnetic field increased the hyperfine splitting of spectral lines, further breaking them up. As the limitation, Bohrs model was unable to explain the experimental evidence.In 1896 a Dutch physics Pieter Zeeman (1865-1943) found that when he placed a source of sodium light betwe en the poles of a strong magnet the lines split into three or more. This could not explain by Bohrs model, The spectral line of some elements can even split to 15 lines. This is called the anomalous Zeeman effect. It cant be explain by that time, and it leads to the new developing of the model to explain it. This startle with the work of de Broglie.The following is the formal definition of Zeeman Effect The splitting of single spectral lines of an emission or absorption spectrum of a substance into three or more components when the substance is placed in a magnetic field. The effect occurs when several electron orbits in the same shell, which normally have the same energy level, have different energies due to their different orientations in the magnetic field. A normal Zeeman Effectis observed when a spectral line of an atom splits into three lines under a magnetic field. Astronomers can use the Zeeman Effect to measure magnetic fields of stars. The following diagrams shows the nor mal spectral line and the Zeeman effect.ReferencesTextbooks1. Excell HSC physics by Neville rabbit warren publish in 20072. Jacaranda Physics Second Edition published in 20043. Bohr and quantum theory by Paul Strathern in19984. Physics Spectrum by Peter H.Eastwell published by McGrathHill in 2000.WebsitesRuthorford scattering experimenthttp//en.wikipedia.org/wiki/Rutherford_scattering last updated 19 July 2008http//library.thinkquest.org/19662/high/eng/exp-rutherford.htmlAtomic structure discovered http//www.neoam.cc.ok.us/rjones/Pages/online1014/chemistry/chapter_8/pages/atomic_structure_discover.htmlFiguresFigure1.Demorcuris http//www.dl.ac.uk/TCS/Software/DL_POLY/ATTIC/DEMOCRITUS/Pictures/democritus.jpgFigure2. Aristotle http//www.bun.kyoto-u.ac.jp/phisci/Images/aristotle.jpgFigure3. John Dalton http//www.learner.org/ course/courses/essential/physicalsci/images/s4.dalton.jpgFigure4. J.J Thomsonhttp//www.wired.com/images/article/full/2008/04/jj_thompson_400px.jpgFigure5.Henri Bec querelhttp//www.mlahanas.de/Physics/Bios/images/HenriBecquerel.jpgFigure6. Ernest Rutherfordhttp//z.about.com/d/chemistry/1/0/n/T/rutherford1.jpgFigure7. Niels Bohrhttp//www.springtimepublishers.com/images/Niels_Bohr.jpg