Alpha particle electronic structure

To explain his observations, Rutherford proposed a new h /pothesis for atomic structure. He suggested that every atom has a tiny central core, called the nucleus, within which all the positive charge and most of the mass is concentrated. Electrons surround this central core, as shown schematically in Figure 2-16. Electrons occupy a volume that is huge compared with the size of the nucleus, but each electron has such a small mass that alpha particles are not deflected by the electrons. Consequently, an alpha particle is deflected only when it passes very near a nucleus, and it bounces back only when it collides head-on with a nucleus. Because most of the volume of an atom contains only electrons, most projectiles pass through the foil without being affected. 

The tracks of alpha particles and of electrons ejected by X rays were first observed in Wilson chambers. Later the more advanced bubble and spark chambers were designed. Another type of detector, which is widely used for recording particle tracks, is one that fixes the changes in the structure of a medium when treated by certain chemical reagents. These are the photoemulsions and the different types of solid detectors.6... 

Rutherford was one of the great scientific figures of the 20th century. He made a number of important discoveries about the structure of atoms and about radioactivity. For example, he found that an atom consists of two distinct parts, the nucleus and the electrons. He also discovered one form of radiation given off by radioactive materials alpha particles. Alpha particles, he found, are simply helium atoms without their electrons. 

If Rutherford and his coworkers had used electrons instead of alpha particles to probe the structure of the nucleus (see Chapter 2), what might have they discovered ... 

For about thirty years this proton-electron combination was believed to be the structure of alpha particles. Similar combinations were believed to make up other massive, positively charged particles. However, this inference created problems. There were theoretical reasons for doubting that the alpha particle could possibly be made up of as many as six smaller particles. 

As you saw in Section 3.2, identification of alpha particles as helium nuclei ( He) and beta particles as electrons played a significant role in our understanding of the structure of the atom. 

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