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Physics - Atoms and Nuclei

Prior to the results of experiments carried out by Geiger and Marsden, who worked under the guidance of Lord Rutherford in 1911, the atom was thought to have the structure of a “plum pudding”. The negatively – charged electrons were pictured as being evenly distributed within a positively – charged uniform mass.

Geiger and Marsden fired alpha particles (α particles) from radon gas at thin gold foil. They then detected the alpha particles after they had been scattered. They found that:

Most of the alpha particles pass through the foil with no deflection

Some are deflected through a range of angles

A small number are “back scattered”.

Rutherford concluded from these results that:

The atom is mainly empty space, allowing most of the alpha particles to pass through undeflected

There are tiny regions of concentrated charge, which explains the large deflection of a small number of the alpha particles

The charge on these regions must be the same sign as that on the alpha particle – positive – to cause the “back scattering”.

Alpha particle – scattering experiments carried out using other materials show that the amount by which an alpha particle is deflected depends on:

The closeness of its approach to the nucleus – the closer it gets, the bigger the force

The charge on the nucleus – the greater the charge, the greater the force of repulsion

The speed of the alpha particle – the faster it travels, the smaller the deflection.

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