The experimental observations which led to the development of the Rutherford model of the atom are reproduced in this program. Students record the number and energy of a-particles scattered at different angles from bombarded thin foils of different pure materials. The observation that many a-particles which bombard a thin foil sample normal to its surface are deflected at angles greater than 25 from normal enables students to understand Rutherford's conclusion that the atom is a small, dense nucleus of positive charge with a surrounding large, diffuse electron cloud. Simple relationships describing electrostatic repulsion between the a-particles and the dense nucleus and the conservation of energy of the a-particles provide the link between Rutherford's theory and experimental evidence which students can verify from their data. (15 minutes)