Abat are isotopes? What is percent natural abundance of isotopes?
Solution : The above figure illustrates Rutherford’s -Scattering experiment. In 1910, Rutherford with help of his -Scattering experiment discovered that almost all the particles passed directly through the gold foil without deflection, with a few particles deflected about 1°, which was consistent with Thomson’s plum-pudding model. Rutherford with help of Ernest Marsden, an undergraduate student working in the laboratory, looked for scattering at large angles for the completeness of result. When observed, a small amount of scattering was observed at large angles and some particles scattered back in the direction they come from. The explanation for these results were clearly inconsistent with Thomson’s plum-pudding model. Rutherford explained the results by postulating the nuclear model of the atom, in which most of the mass of each gold atom and all of its positive charge reside in a very small, extremely dense region which he called the nucleus. He postulated further that m ost of the volume of an atom is empty space in which ele ctrons move around the nucleus. Subsequent experiments led to the discovery of positive particles (protons) and neutral particles (neutrons) in the nucleus. Thus, it was discovered that the atom is composed of electrons, protons, and neutrons. In the - scattering experiment, most of the particles passed through the foil were unscattered because they did not e ncounter the m inute nucleus of any gold atom. Occasionally, however, an - particle c ame close to a gold nucleus. In such encounters, the repulsion between the highly positive charge of the gold nucleus and the positive charge of the - particle was strong enough to deflect the particle, as shown in Figure above. Rutherford interpretation to the following observations made during his -particle scattering experiments: (a) Most - particle were not appreciably deflected as they passed through the gold foil : In Rutherford - particle s cattering experiment, most - particle were not appreciably deflected as passed through the gold foil. By this observation he explained that there is an empty space around the nucleus in an atom. In this empty space electrons moves around the nucleus. (b) A few - particle were deflected at very large angles : When Rut herford, looked for scattering of - particle t large angles, a small amount of scattering was observed and some particles scattered back in the direction they come from. The - particle are positively charged in the nuclear model of the atom. The mass of gold atom and its positive charge reside in a very small extremely dense region, called nucleus. These few - particle are deflected at very large angles because of repulsion by very small positively charged nucleus. (c) What differences would you expect if beryllium foil were used instead of gold foil in the - particle scattering experiment If beryllium(Be) foil were used instead of g old(Au) foil in the - particle scattering experiment, then only few - particle were scattered and few were strongly scattered back from where they came from. This is because of the nuclei of Be atom. Nuclei of Be are much smaller in volume than the nuclei of Au. Also, Be has smaller positive charge at the nucleus than Au. Hence, only few - particle are scattered and few are strongly scattered back when B e foil is used instead of Au foil.