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Chapter one: Atomic Models

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الكلية كلية التربية للعلوم الصرفة     القسم قسم الفيزياء     المرحلة 3
أستاذ المادة فؤاد عطية مجيد       07/11/2016 21:33:26
Chapter one: Atomic Models
1.1 Thomson’s model
Based on these concepts Thomson developed a model for the atom consisting of the electrons as negatively charged particles of low mass and some substance that should carry positive charge and nearly all the mass within the atom. Since the elements were arranged according to their mass number A, the atoms were thought to consist of A positive particles and A electrons in a structure as shown below. Note that the atomic number Z does not play a role yet.

1.2 Rutherford scattering
Rutherford scattering is a phenomenon that was explained by Ernest Rutherford in 1911, and led to the development of the orbital theory of the atom. It is now exploited by the materials analytical technique Rutherford backscattering. Rutherford scattering is also sometimes referred to as Coulomb scattering because it relies on static electric (Coulomb) forces.
Highlights of Rutherford’s Experiment:
• A beam of ? particles were aimed at a thin gold foil.
• Most of the particles passed through without deflection.
• Others were deflected by various angles.
• Some were backscattered.

From these results Rutherford concluded that the majority of the mass was concentrated in a minute, positively charged region (the nucleus) surrounded by electrons. When a (positive) alpha particle approached sufficiently close to the nucleus, it was repelled strongly enough to rebound at high angles. The small size of the nucleus explained the small number of alpha particles that were repelled in this way. Rutherford showed,using the method below, that the size of the nucleus was less than about 10–14 m .


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