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Atomic models

الكلية كلية التربية للعلوم الصرفة     القسم قسم الفيزياء     المرحلة 3
أستاذ المادة فؤاد عطية مجيد       01/11/2014 15:41:19
Historical perspective


In the years before Bohr formulated his theory of the atom, based upon the principles of quantum physics, some steps had been made on the understanding of the atomic structure. We list here some important contributions:

? Mendeleev had developed a concept for arranging the known chemical elements based on their mass. Order was given in terms of increasing mass, while the elements were further arranged according to the ordering principle of chemical behavior. The columns in the table of the elements relate to chemical valence. In 1869 the Periodic Table was not yet complete. ? Avogadro had conceived the idea that gasses consist of discrete particles and had established the law that equal volumes of gas at equal pressure and temperature contain the same number of such particles, although the actual number was not yet determined. Avogadro s hypothesis was not accepted by many physicists for a long time. ? It was realized for some time that electrostatic charges were important for the building blocks of matter. This followed from Faradays experiments on electrolysis , from which it was deduced that ions move in a liquid as charged particles, and from the experiments on radioactivity in which electrically charged particles were emitted.

? Thomson s experiments on cathode rays were important for the determination of some properties of the constituents of matter. The charged particles, emitted from a cathode, were deflected in a combination of crossed static electric and magnetic fields,
and detected on the phosphorent screen (see Fig). Hence impinging charged particles could be made visible by the light emitted by the screen.

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.


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