Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Hydrogen atom wikipedia , lookup
Fundamental interaction wikipedia , lookup
Elementary particle wikipedia , lookup
Electrostatics wikipedia , lookup
Standard Model wikipedia , lookup
History of subatomic physics wikipedia , lookup
Electric charge wikipedia , lookup
Nuclear physics wikipedia , lookup
Atomic nucleus wikipedia , lookup
Thomson’s discovery of the electron and the “Plum pudding model” Sir Joseph John “J. J.” Thomson, (18 December 1856 – 30 August 1940) was a British physicist and Nobel laureate. He is credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer. Thomson was awarded the 1906 Nobel Prize in Physics for the discovery of the electron and for his work on the conduction of electricity in gases. Discovery of the electron Until 1897, scientists believed atoms were indivisible; the ultimate particles of matter, but Thomson proved them wrong when he discovered that atoms contained particles known as electrons. Thomson discovered this through his explorations on the properties of cathode rays. Thomson found that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). He concluded that these rays, rather than being waves, were composed of very light negatively charged particles which he called "corpuscles". (Later scientists preferred the name electron which had been suggested by George Johnstone Stoney in 1894, prior to Thomson's actual discovery). Thomson believed that the corpuscles emerged from the atoms of the trace gas inside his cathode ray tubes. He thus concluded that atoms were divisible, and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the plum pudding model as the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary). Experiments with cathode rays Earlier, physicists debated whether cathode rays were immaterial like light ("some process in the aether") or had mass and were composed of particles. The aetherial hypothesis was vague, but the particle hypothesis was definite enough for Thomson to test. First experiment While supporters of the aetherial theory accepted the possibility that negatively- charged particles are produced in Crookes tubes, they believed that they are a mere byproduct and that the cathode rays themselves are immaterial. Thomson set out to investigate whether or not he could actually separate the charge from the rays. Thomson constructed a Crookes tube with an electrometer set to one side, out of the direct path of the cathode rays. Thomson could trace the path of the ray by observing the phosphorescent patch it created where it hit the surface of the tube. Thomson observed that the electrometer registered a charge only when he deflected the cathode ray to it with a magnet. He concluded that the negative charge and the rays were one and the same. Second experiment Thomson's illustration of the Crookes tube by which he observed the deflection of cathode rays by an electric field. Cathode rays were emitted from the cathode C, passed through slits A (the anode) and B (grounded), then through the electric field generated between plates D and E, finally impacting the surface at the far end. The cathode ray (blue line) was deflected by the electric field (yellow). In his second experiment, he investigated whether or not the rays could be deflected by an electric field. Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because their tubes contained too much gas. Thomson constructed a Crookes tube with a near-perfect vacuum. At the start of the tube was the cathode from which the rays projected. The rays were sharpened to a beam by two metal slits - the first of these slits doubled as the anode, the second was connected to the earth. The beam then passed between two parallel aluminium plates, which produced an electric field between them when they were connected to a battery. The end of the tube was a large sphere where the beam would impact on the glass, created a glowing patch. Thomson pasted a scale to the surface of this sphere to measure the deflection of the beam. When the upper plate was connected to the negative pole of the battery and the lower plate to the positive pole, the glowing patch moved downwards, and when the polarity was reversed, the patch moved upwards. Third experiment In his third experiment, Thomson measured the mass-to-charge ratio of the cathode rays by measuring how much they were deflected by a magnetic field and how much energy they carried. He found that the mass to charge ratio was over a thousand times lower than that of a hydrogen ion (H+), suggesting either that the particles were very light and/or very highly charged. Conclusions As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter. —J. J. Thomson As to the source of these particles, Thomson believed they emerged from the molecules of gas in the vicinity of the cathode. If, in the very intense electric field in the neighbourhood of the cathode, the molecules of the gas are dissociated and are split up, not into the ordinary chemical atoms, but into these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and projected from the cathode by the electric field, they would behave exactly like the cathode rays. —J. J. Thomson Thomson imagined the atom as being made up of these corpuscles swarming in a sea of positive charge; this was his plum pudding model. This model was later proved incorrect when Ernest Rutherford showed that the positive charge is concentrated in the nucleus of the atom. Plum pudding model The plum pudding model of the atom, was proposed in 1904 before the discovery of the atomic nucleus. In this model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively-charged "plums" surrounded by positively-charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly A schemic presentation of the plum pudding model of the atom. In Thomson's mathematical model the "corpuscles" (or modern electrons) were arranged non-randomly, in rotating rings. rotating rings of electrons (see below). Instead of a soup, the atom was also sometimes said to have had a "cloud" of positive charge. With this model, Thomson abandoned his earlier "nebular atom" hypothesis in which the atom was composed of immaterial vorticies. Now, at least part of the atom was to be composed of Thomson's particulate negative corpuscles, although the rest of the positively-charged part of the atom remained somewhat nebulous and ill-defined. The 1904 Thomson model was disproved by the 1909 gold foil experiment, which was interpreted by Ernest Rutherford in 191 to imply a very small nucleus of the atom containing a very high positive charge (enough to balance about 100 electrons in gold), thus leading to the Rutherford model of the atom. Finally, after Henry Moseley's work showed in 1913 that the nuclear charge was very close to the atomic number, Antonius Van den Broek suggested that atomic number is nuclear charge. This work had culminated in the solar-system-like (but quantum-limited) Bohr model of the atom in the same year, in which a nucleus containing an atomic number of positive charge is surrounded by an equal number of electrons in orbital shells. Thomson's model was compared (though not by Thomson) to a British dessert called plum pudding, hence the name. Thomson's paper was published in the March 1904 edition of the Philosophical Magazine, the leading British science journal of the day. In Thomson's view: ... the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, … In this model, the electrons were free to rotate within the blob or cloud of positive substance. These orbits were stabilized in the model by the fact that when an electron moved farther from the center of the positive cloud, it felt a larger net positive inward force, because there was more material of opposite charge, inside its orbit (see Gauss's law). In Thomson's model, electrons were free to rotate in rings which were further stabilized by interactions between the electrons, and spectra were to be accounted for by energy differences of different ring orbits. Thomson attempted to make his model account for some of the major spectral lines known for some elements, but was not notably successful at this. Still, Thomson's model (along with a similar Saturnian ring model for atomic electrons, also put forward in 1904 by Nagaoka after James C. Maxwell's model of Saturn's rings), were earlier harbingers of the later and more successful solar-system-like Bohr model of the atom. Sources: http://en.wikipedia.org/wiki/J._J._Thomson http://en.wikipedia.org/wiki/Plum_pudding_model