J.J.Thomson's model of an atom

It shows J.J.Thomson's contribution in the field of atoms

1. THOMSON'S MODEL OF an ATOM

2. J.J.THOMSON Joseph John Thomson was born in 1856 in Cheetham Hill, Manchester, England. One of Thomson's greatest contributions to modern science was in his role as a highly gifted teacher: seven of his research assistants and his son won Nobel Prizes in physics. He was awarded a Nobel Prize in 1906, "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." He was knighted in 1908 and appointed to the Order of Merit in 1912. In 1914 he gave the Romanes Lecture in Oxford on "The atomic theory“.. He died on 30 August 1940 and was buried in Westminster Abbey, close to Sir Isaac Newton.

3. J.J.THOMSON’S CONTRIBUTION TO ATOM Discovery of Electron. Plum Pudding model Experiments with cathode rays. Isotopes and mass spectrometry .

4. DISCOVERY OF ELECTRON . Thomson, in 1897, was the first to suggest that the fundamental unit was over 1000 times smaller than an atom, suggesting the subatomic particle now known as the electron. He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1000 times lighter than the hydrogen atom, but also that their mass was the same in whichever type of atom they came from.

5. PLUM PUDDING MODEL

6. PLUM PUDDING MODEL Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruit) in a spherical Christmas pudding . We can also think of a watermelon , the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.

7. OBSERVATION BY J.J.THOMSON An atom consist of a positively charged sphere and the electrons are embedded in it. The negative and positive charges are equal in magnitude .So, the atom as a whole is electrically neutral.

8. DRAWBACKS OF PLUM PUDDING MODEL Thomson attributed that the mass of an atom due to electrons and protons are evenly spread throughout the atom. This is not in agreement with the observations of Rutherford who concluded that mass is concentrated in a very small space in the center of the atom later called nucleus.

9. CATHODE RAY TUBE . The cathode ray tube (CRT) is a vacuum tube containing one or more electron guns (a source of electrons or electron emitter) and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam(s) onto the screen to create the images. The images may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets or others. CRTs have also been used as memory devices, in which case the visible light emitted from the fluorescent material (if any) is not intended to have significant meaning to a visual observer (though the visible pattern on the tube face may cryptically represent the stored data).

10. Cathode: The cathode is the negatively charged electrode. The cathode attracts cations or positive charge. The cathode is the source of electrons or an electron donor. It may accept positive charge. Anode: The anode is the positively charged electrode. The anode attracts electrons or anions. The anode may be a source of positive charge or an electron acceptor. IMPORTANT TERMS TO BE LEARNT

11. THOMSON’S EXPERIMENT WITH CATHODE RAY DISCHARGE TUBE Experiments on the magnetic deflection of cathode rays: Thomson first investigated the magnetic deflection of cathode rays. Cathode rays were produced in the side tube on the left of the apparatus and passed through the anode into the main bell-jar, where they were deflected by a magnet. Thomson detected their path by the fluorescence on a squared screen in the jar. He found that whatever the material of the anode and the gas in the jar, the deflection of the rays was the same, suggesting that the rays were of the same form whatever their origin.

12. Experiment to show that cathode rays were electrically charged: The cathode ray tube by which JJ Thomson demonstrated that cathode rays could be deflected by a magnetic field, and that their negative charge was not a separate phenomenon. 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 THOMSON’S EXPERIMENT WITH CATHODE RAY DISCHARGE TUBE

13. 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 aluminum 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 . Cathode rays could be deflected electrically

14. A . The charge of an electron is negative. B. The mass of an electron is related to its charge. C. Cathode rays are always negatively charged particles. D. Cathode rays are made of electrons regardless of the gas used OBSERVATION BY J.J.THOMSON

15. DRAWBACKS OF CATHODE RAY DISCHARGE TUBE EXPERIMENT: The weight of an electron is about a thousandth part of a hydrogen atom, it would mean that a single atom, especially of the heavier elements, would contain many thousand electrons. But J.J. Thomson himself found that the number of electrons in an atom cannot be greatly different from the atomic weigh. According to this model, hydrogen can give rise to only one spectral line, contrary to the observed fact of several lines. This model could not explain the large angle scattering of alpha particles by thin metal foils .

16. DONE BY: RAHUL SRIVATSAN.M.R. VISHWANATH.A. DHANRAJ.D. SANJAY.S. GANAPATHY.M. ABHINAV.R. SANTHOSH.S. VIVEK.S. VARUN PRASATH.B. AKASH.L.K.

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