13(T)%20-%20Atoms - 13 ATOMS Page 1 Introduction i In 1895...

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13 - ATOMS Page 1 Introduction i ) In 1895, Jean Perin proved that electrons ( known as cathode rays that time ) are negatively charged. ii ) Shortly thereafter, J. J. Thomson determined charge ( e ) to mass ( m ) ratio of electrons to be of the order of 10 11 . This meant that m / // e of electron is of the order of 10 - 11 and that the mass of electron is very small. iii ) In 1909, Millikan measured the magnitude of the charge of an electron. iv ) Studies on X-rays, discovered in 1895, resulted in the discovery of radioactivity. v ) Rutherford’s experiments on radioactivity proved emission of α -particles besides electrons in radioactive radiations. Thus one more particle was discovered. vi ) In the 19th century, scientists were trying to measure wavelengths of radiations emitted by different gases filled in discharge tubes using the diffraction grating discovered by Henry Rowland. These wavelengths were found to be discrete and dependent on the type of gas filled in the discharge tube. vii ) Same time Max Planck presented the photon theory and showed that the black body radiation is discrete. Einstein explained photoelectric effect using photon theory of light for which he received the Nobel prize. viii ) In 1902, J. J. Thomson presented an atomic model according to which positive charge is distributed uniformly in a small spherical space of atom and electrons are embedded inside it like the seeds of watermelon embedded in its pulp. Hence, the model was called watermelon model or plum pudding model. The magnitude of positive charge was taken equal to the total negative charge of electrons to explain electrical neutrality of atom. But, the electrons embedded in the uniform distribution of positive charges should experience a force towards the centre of the atom directly proportional to the distance from the centre. Hence, they should perform either SHM or uniform circular motion. As both these motions are accelerated, electrons should emit continuous radiation according to the electromagnetic theory of Maxwell. This made it difficult to understand the emission of discrete wavelengths from atoms. Also such a model of atom cannot form a stable structure. To overcome these problems, Thomson assumed the charges to remain stationary unless disturbed from outside and thought about different arrangement of electrons in different atoms. He also estimated the size of atoms to be of the order of 10 - 8 cm from the wavelengths of radiations emitted. Despite all these efforts, he could not explain why the radiation consisted of discrete wavelengths. ix ) In 1906, Rutherford observed that α -particles passing through a slit provided in the chamber and incident on a photographic plate do not give sharp image of the slit. But on evacuating the chamber, the image became sharp. From this, he concluded that the α -particles must be scattered by the air particles in the chamber. 13.1
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This note was uploaded on 11/28/2011 for the course PHYSICS 300 taught by Professor Smith during the Spring '06 term at ITT Tech Flint.

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13(T)%20-%20Atoms - 13 ATOMS Page 1 Introduction i In 1895...

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