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structure of atom 33 Table 2.1 Properties of Fundamental Particles Name Symbol Absolute Relative Mass/kg Mass/u Approx. charge/C charge mass/u –19 –31 Electron e – 1.602176×10 –1 9.109382×10 0.00054 0 –19 –27 Proton p + 1.602176×10 +1 1.6726216×10 1.00727 1 –27 Neutron n 0 0 1.674927×10 1.00867 1 Different atomic models were proposed In the later half of the nineteenth century to explain the distributions of these charged particles in an atom. Although some of these different kinds of rays were discovered, models were not able to explain the stability besides those mentioned earlier. Wilhalm of atoms, two of these models, one proposed Röentgen (1845-1923) in 1895 showed by J.J. Thomson and the other proposed by that when electrons strike a material in Ernest Rutherford are discussed below. the cathode ray tubes, produce rays which can cause fluorescence in the fluorescent 2.2.1 Thomson Model of Atom materials placed outside the cathode ray J. J. Thomson, in 1898, proposed that an tubes. Since Röentgen did not know the atom possesses a spherical shape (radius approximately 10 –10m) in which the positive nature of the radiation, he named them X-rays and the name is still carried on. It was charge is uniformly distributed. The electrons noticed that X-rays are produced effectively are embedded into it in such a manner as to give the most stable electrostatic arrangement when electrons strike the dense metal anode, (Fig. 2.4). Many different names are given calledtargets. These are not deflected by the electric and magnetic fields and have a very to this model, for example, plum pudding, raisin pudding or watermelon. This model high penetrating power through the matter and that is the reason thatthese rays are used to study the interior of the objects. These rays are of very short wavelengths (∼0.1 nm) and possess electro-magnetic character (Section 2.3.1). Henri Becqueral (1852-1908) observed that there are certain elements which emit radiation on their own and named this Fig.2.4 Thomson model of atom phenomenon as radioactivity and the can be visualised as a pudding or watermelon elements known as radioactive elements. of positive charge with plums or seeds This field was developed by Marie Curie, (electrons) embedded into it. An important Piere Curie, Rutherford and Fredrick Soddy. feature of this model is that the mass of the It was observed that three kinds of rays i.e., atom is assumed to be uniformly distributed α, β- and γ-rays are emitted. Rutherford over the atom. Although this model was able found that α-rays consists of high energy to explain the overall neutrality of the atom, particles carrying two units of positive charge but was not consistent with the results of later and four unit of atomic mass. He concluded experiments. Thomson was awarded Nobel that α- particles are helium nuclei as when α- Prize for physics in 1906, for his theoretical particles combined with two electrons yielded and experimental investigations on the helium gas. β-rays are negatively charged conduction of electricity by gases. Reprint 2026-27