1. In 1911 Rutherford proposed the nuclear model of atomic structure. He suggested that an atom consists of a central nucleus (where most of the mass of the atom is concentrated) having a positive charge, surrounded by moving electrons carrying negative charge. Geiger and Marsden carried out an experiment to verify his proposal. The Geiger/Marsden a Particle Scattering Experiment
1. The apparatus is illustrated in the diagram below.
2. The apparatus was in an evacuated container. The detector was a ZnS screen observed through a low power microscope. Each time an alpha particle hit the screen, a small flash of light was produced. 3. The detector was mounted on a support such that it could be rotated to measure the angular deflection of the alpha particles as they passed through a very thin sheet of gold. They measured the numbers of particles deflected through various angles. 4. It was found that most of α particles pass through the gold undeflected; only a relatively small number are deflected (scattered). 5. Their results were considered to confirm Rutherford’s model and allowed them to estimate the size of the nucleus (greater than 10-14m) and the size of the atom (greater than10-10m), thus producing the slightly surprising conclusion the most of the space occupied by an atom is empty space!
Closest Approach of an Alpha Particle to a Nucleus
1. For a given speed of alpha particle, the closest approach to a nucleus, rmin, will occur when the initial direction of motion of the particle is along the line joining the centers of particle and nucleus. |
2. In this case, at the point of closest approach, the speed of the particle is zero. 3. As the particle approaches the nucleus, kinetic energy is being converted to electrical potential energy. K.E. lost = E.P.E. gained|
4. Electrical potential at a distance r from a point charge Q is given by |
5. For a nucleus of atomic number...