【文章內容簡介】 uring the increase in mass of the cathode. Experiments can be carried out with different elements and results confirm the atomic theory and the theory of valence. Most interesting for our discuss is the calculation of the mass of an atom of hydrogen, the lightest element. This turns out to be 1027 kg, approximately 1800 times that of an electron. Knowing atomic masses, and the density of materials, it is straightforward to obtain values for atomic dimensions. The only problem is that unless the atoms in a sample of material are arranged in a regular pattern, the answer is not very meaningful. For crystalline substances, Xray diffraction enable the arrangement of atoms to be discovered. The dimensions of the crystal structure can then be calculated. Fig. A single cell of the simple cubic lattice of sodium chloride. The lattice is held together by the attraction between the positively charged sodium ion and the negatively charged chlorine ion. For example, crystals of rock salt (sodium chloride, NaCl) are found to have a cubic structure, with sodium and chlorine ions on alternate corners (Fig. ). If M is the kilogram molecular weight of NaCl and ρ the density of the crystal, the volume of one kgmolecule is /VM?? There are 2N atoms is one kgmolecule, where N is Avogadro’s number. Therefore the distance between the centres of atoms, d is given by: 3 / (2 )d M N?? For sodium chloride, this works out as 1010m and similar results are obtained for other crystals. Of course, such calculations only tell us the distance between the centres of the atoms and hence the maximum possible size for an atom. To go further, it is necessary to investigate the structure of the atom itself. THE NUCLEAR MODEL OF THE ATOM Fig. Classical models of the atom. (a) Thomson’s model. Small, negatively charged electrons are held in a dense, positively charged body. (b) Rutherford’s model. The vast majority of the mass and all the positive charge are concentrated in a relatively tiny nucleus, surrounded by electrons. In both pictures the size of the electrons and of the nucleus are exaggerated. The nucleus should be at least 1000 times smaller and the electrons many times smaller again. In order to explain the result, Rutherford proposed a new model in which all the positive charge and most of the mass of the atom resided in a central nucleus, surrounded by electrons orbiting in free space. The size of the nucleus would be small pared with the size of the atom (Fig. (b)). This model would give a qualitative explanation for Geiger and Marsden’s results as most of the α particles would pass through the atom without encountering any matter, but a very few would collide with the massive nucleus. However, much more importantly, this model gives a precise quantitative agreement between theory and experiment. Because of the seminal nature of this model, it is worthwhile looking at Rutherford’s analysis in detail. Only classical of physics is required . Fig Path of α particle (charge +2e) in the field of the nucleus (charge +Ze). The nucleus is at the origin and is very much more massive than the α particle. The force F is due to electrostatic repulsion. The analysis of the scattering experiment falls into two parts. First, it it necessary to obtain an expression for the deflection of a single α particle as a function of its kiic energy and its trajectory relative to the nucleus. The particle and the nucleus are assumed to be very small, and t