27.1.3 Observe and explain Alpha particles are elementary particles emitted by s

Question

27.1.3 Observe and explain Alpha particles are elementary particles emitted by some materials, such as uranium; an alpha particle has an electric charge of +2e and the mass of a helium atom. To help determine atomic structure, Rutherford and his graduate students shot alpha particles at a thin sheet of mica or metals. They looked at places where the particles hit a fluorescent screen after passing through the thin sheets. They also looked for alpha particles that possibly bounced backward off the thin sheet (on the same side as the incoming alpha particles). They observed that most alpha particles moved through the sheet with minor deflection. However, a small frac- tion of the alpha particles bounced backward. At this time physicists already knew that atoms are electrically neutral and contain several electrons. Electrons were known to have a very small mass much smaller than the mass of the atom. What could Rutherford suggest about the distribution of the positive charge in the atom to explain his experiments?

Explanation / Answer

Distribution of the positive charge in an atom:

We already know that

1. two charges of same type repel each other whereas opposite charges attract each other.

2. Atoms are electrically neutral and contain several electrons, having small mass.

Using 1, Rutherford could suggest that the small fraction of alpha particles reflected due to positive charge and the minor deflections are due to electrons .

In case of reflection , since electrons are very small compared to the atoms, reflection of the atoms has to be due to group of positive charges concentrated at one area and minor deflections may be due to the electrons scattered around the positive charge area.

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