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The Bohr Model

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The Bohr Model TTHHEE ATATOMOMICIC MMODELODEL A historical comparison Early Greek Models There were two main competing models: ARISTOTLE’s Four DEMOCRITUS’s Smallest Element Model Particle Model •all matter was composed •matter can be broken of combinations of air, down into smaller particles; water, fire and earth. each type of substance has different types of particles DALTON Did various quantitative studies and proposed: •that all matter is composed of small, solid, indivisible particle spheres called atoms •all atoms of the same element are identical but different elements had different particles •atoms can combine in specific whole number ratios The BILLIARD BALL Model THOMSON •Used gas discharge tubes to identify that there is a negative particle in the atom •Proposed a model that included negative electrons embedded in a positive sphere •The Raisin Bun or Plum-Pudding Model Don’t forget Nagaoka RUTHERFORD •Was testing Thomson’s model using alpha rays and a gold foil. •Found a difference in his prediction and the results Review the Rutherford Model The atom has a dense, positively charged centre called a nucleus Around the atom is a large region of empty space Negatively charged electrons can be found orbiting the nucleus in the empty space Flaw in Rutherford’s Theory •as electrons move about the nucleus, according to classical physics, they will emit electromagnetic radiation and lose energy •this loss of energy would cause the electrons to slow down and eventually fall into the nucleus (atom collapses!) •since the above do not occur, a new theory was needed Bohr discovers that... Energized gases emit coloured light Different types of gases emit different colours of light Through a spectroscope specific bands of colour can be seen Each colour of light is related to a specific amount of energy The colours of light emitted are related and can be predicted by looking at the number of electrons in an atom Electrons are found in energy levels the nucleus is a dense, positive sphere a series of shells or energy levels or orbits surround the nucleus each level is slightly larger than the previous and is related to more energy Energy levels... en e h rg av y electrons are found on e lev elec up el t 2 tro o 2( ca particular energy levels ns 2 n ) 2= 8 electrons may jump from one level to another but they may not travel between levels ener gy lev each energy level can has el 1 2( 2 larger 1) e energy lect hold a specific number of but levels rons may be may e 2 empty xist electrons (2n ) Changing levels... light is released electrons can jump to higher levels by add energy lose energy absorbing energy when it loses that energy, it will return to its previous level the energy released produces the line spectrum Bohr observed The Hydrogen atom • Bohr observed the following bright line spectrum of hydrogen and was able to make a quantitative connection between each coloured line and a specific electron jump. Ele ctr n j on tro ump lec ing E ng from Electron jumping mpi l leve Electron jumping ju eve 2 t l from level 2 to m l o lev from level 2 to fro vel and el 6 level 4 and back o le k bac level 5 and back 2 t bac k nd Released energy produces coloured light related to how3 m auch energy is released Line spectra correspond to the energy released What about the jump from level 1 to level 2? The successes & failures of Bohr’s Theory •Bohr’s Theory was formulated from a large body of research about the nature of light and energy ==> this large body of evidence resulted in the Quantum Theory •Bohr’s theory explain the Bright line spectrum of hydrogen very well, but did not work for more complex atoms or ions •also the scientists (e.g Albert Michelson in 1891) found that the main lines of the bright line spectrum of hydrogen were actually composed of more than one lines •this led Arnold Sommerfeld (1915) to propose that Bohr’s main energy levels/shells consisted of subshells or sublevels •by the 1920s Bohr’s planetary model had to be abondoned. The Quantum (wave) Mechanics Model Where the Bohr model visualized electrons travelling around the nuclues in orbits of fixed radius, the modern theory views the atom as: – a positively charged nucleus surrounded by pulsating waves ==>proposed by Louis de Broglie ==> Clinton Davisson and G.P.Thomson obtained experimental confirmation ==> Erwin Schrodinger theorized that electrons can only release/absorb certain (quantized) energies because waves can only have whole number of wavelengths; electrons must behave as waves; he developed a wave equation to describe the energy and motion of an electron; ==> in solving the equation, one can deduce the probability of finding the electron in a certain position around the nucleus The Quantum (wave) Mechanics Model Con’t it is impossible to simultaneously know the speed and exact location of an electron (Heisenberg uncertainty principle) Exactly how electrons move in an atom is not known..in terms of location all that is known is the probability of finding an electron in a certain region (electron probability density) The regions of space where electrons occupy are called orbitals (also called electron clouds), so unlike the Bohr model which uses orbits to pinpoint where electrons are, this theory uses the term orbital..orbitals differ in size, shape, and space orientation.
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