Most of the alpha particles are only slightly deflected, as expected, but occasionally one is deflected back towards the source. The above diagram shows what we would expect the result of Rutherford's experiment to be if the "plum pudding" model of the atom is correct. However, once in a while he observed an alpha particle that was scattered right back towards the radioactive source. And in fact the results of Rutherford's experiments usually followed this model: almost all of the alpha particles emerged on the other side slightly deflected by their interaction with the gold. If Thomson's plum pudding model was correct, the experiments would be sort of similar to firing BB's from a BB gun into a thin slab of cream cheese with chives. He was directing a beam of these alpha particles onto a very thin piece of gold foil. He knew that these particles had a mass much larger than the electron and had a net positive electric charge now we know that these particles are identical to the nucleus of the helium atom. In the early twentieth century Rutherford was experimenting with one of the newly discovered radioactive substances, one that emitted alpha particles. Hydrogen, for example, has one electron helium has two carbon has six, etc. The number of electrons determines the particular chemical element. This was called the plum pudding model of the atom. The total charge of the electrons exactly balanced the positive charge of the large mass, so the total electric charge was zero. Thus he proposed that atoms consisted of a large massive positively charged body with a number of small negatively charged electrons scattered throughout it. He determined that these electrons had a negative electric charge and compared to the atom had very little mass. Throughout the nineteenth century atomism became an idea that came to dominate thought in a number of fields, including political science, sociology, psychology, biology and more. Nonetheless opposition from critics such as Mach, who never believed in atoms, was largely ignored. Dalton "proved" his theory with a number of assumptions, each of which is either factually wrong or was used in a logically inconsistent manner. This idea was totally counter-intuitive and required a new physics.\)īy the late nineteenth century, most people had accepted Dalton's proposal of 1808 that matter was made of atoms. The releasing of the photons is responsible for the line spectra given off by elements when excited. Then the electrons would release the photons in quantized amounts to return to ground state. Why?īohr took the ideas of Rutherford (nuclear model), Planck (quanta), Einstein (photoelectric effect and spectroscopy and created the Planetary Model.īohr proposed that electrons were in energy levels (ground state) and absorbed photons of certain frequencies to move to a higher energy level (excited state). Classical physics says that opposites attract and likes repel, so the negative electrons should be attracted to the positive nucleus. Niels Bohr change the atomic theory by realizing that the electrons did not crash into the nucleus as would be expected in classical physics.
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