History of Atomic Theory

History of Atomic schemeThe sectionic theory is developed since 2000 years ago the Greek philosopher Democritus proposed that on that point was a limit to how sm al ace nonp beil could be divide issuance, this flyspeckest field of study indivisible p obligate was called p border. However this miteic theory of Democritus was criticized by Aristotle who proposed a simulate based on four basic elements of earth, air, fire and water.Aristotles sensible horizon held for the next 2000 years as it better suited religious beliefs of the time. In 1801, an English t to each one(prenominal)er named John Dalton proposed his particleic theory which stated matter is composed of all small indivisible fractions, elements contain one character of atom different elements contain different atoms, compounds contain much than one type of atom. In 1904, British physicist J.J Thomson and others demonstrated that cathode rays ( negatrons) were present in all matter.Thomson proposed that the atom was a sphere of positive charge in which embed were rings of negative charges (electrons) , like plums in a pudding. Lather on, in 19 century, the discoveries of radiation leads to a new progress of physics. We knew that certain elements emitting radiation, this suggested that atoms atomic number 18 no longer indivisible and not indestructible, as proposed in Daltons atomic pretense.Radiation provides an main(prenominal) tool for the study of matter. In 1911, Rutherford first suggested the exercise of of import particles to probe the internal structure of the atom. Finally, the nucleus and its protons were ascertained.Rutherfords dispersal tasteErnest Rutherford, directed an experiment to Hans Geiger and Ernest Marsden in 1909, in which the newly discovered of import particles (Helium nuclei) were fired at a thin gold foil socio-economic class 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).Most 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 poseur of the atom current at the time. About 1 in 8000 was deflected through angles great than 90 degree. The result was so unexpected that Rutherford was very unexpected that Rutherford was promoted to keep.it was almost incredible as if you fired a 15 inch shell at a piece of tissue and it came back and peach you.Therefore Rutherford concluded that majority of the mass of the atom was concentrated in the nucleus. The small size of the nucleus relieveed 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(prenominal) than about 10 -14m.Bohrs computer simulation, how it fit experimental observationAccelerated electrons emitted and lose efficiency which predicted by Predicted by Maxwell and co nfirmed by Hertz, merely the electron doesnt whirl into the nucleus be consume of power losing.Later on, In 1903.H.G.J Morseley found simple, regular consanguinity betwixt the frequencies of X-ray discharge aura, thus it provided an evidence to support Bohrs model. Scientist began to snuff it on an alternating model to replace it.Bohrs contestsIn 1913 the Danish physicist Niels Bohr (18851992), ramble forward nigh radical propositions to reputation for the discrepancies between Rutherfords model of the atom and the available experimental evidence. Bohrs postulates are1. Electron smoke remove in certain allowed orbitsstationary states ( button) without radiating aptitude.2.when an electron falls from a higher energy direct to a lower energy train, it emits energy that is quantised by the dining table relationship E2 E 1= hf.3. Augular momentum (mvr) is quantised and can only issue values 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 un cognise. They comprise was a fact.The second postulate explains the telephone circuit emission spectra. Emission (or absorption) of force is discontinuous and corresponds to a transition between twain stationary states. Since the energy can be quantized, the emission, the frequency of the emitted (or absorbed) radiation is predetermined. A transition between different states will lead to difference frequencies or colours.The third postulate instalively sets limits on the radius of the allowed orbits.Bohrs modelBohr realised that if his model was correct, each atom would substantiate a ghostlike fingerprint to the differences between electron energy take in that atom. The Rydberg equation which also known as Balmer equation, has given him evidence for the quantised emission of the energy from total heat atom, star him to going on to further his model and define his postulate.So the total heat spectrum was very significant to the development of Bohrs model of the atom, because without the understanding of it, Bohr may not continue his compute of the model.Produced and observable when hydrogen gas was excited 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 whole number values of n, only as suggested by Bohr in his postulates.Quantum number and quantum changesIts contingent to determine the energy of each orbit using Bohrs model and from this piss 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 wad 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 energy 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. concord to Bohr, the Balmer series (shown on the bottom of the diagram as the hydrogen spectrum) was cause by changing energy levels, in the process releasing pass. As shown, larger energy changes produce more energetic photons, a s 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 discoveries.)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 final 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 natural philosophy 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 mix ture 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 intense 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 thin spectral lines could not be explained. The splitting of spectral lines when the sample was located in a magnetized field (called the Zeeman effect, and discussed below) could also not be explained.The postulates faced a problem 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 satellite electrons are shielded from the nucleus by the inner 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 noted 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 elation spectroscopes improved 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 aftermathZeeman Effect occurred when a magnetic field us pass through a discharge tube. The magnetic field increase the hyperfine splitting of spectr al 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 between 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 begin 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 alike shell, which normally have the same energy level, have different energies due to their different orientations in the magnetic field. A nor mal 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 normal spectral line and the Zeeman effect.ReferencesTextbooks1. Excell HSC physics by Neville Warren published in 20072. Jacaranda Physics Second Edition published in 20043. Bohr and quantum theory by capital of Minnesota 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/phis ci/Images/aristotle.jpgFigure3. John Dalton http//www.learner.org/channel/courses/essential/physicalsci/images/s4.dalton.jpgFigure4. J.J Thomsonhttp//www.wired.com/images/article/full/2008/04/jj_thompson_400px.jpgFigure5.Henri Becquerelhttp//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