## What is Rutherford’s scattering experiment?

Rutherford’s scattering experiment is also known as the gold foil experiment. This experiment disproved the plum pudding model of J.J. Thomson. In a gold foil experiment, high-energy alpha particles are made to strike a thin gold foil. The scattered alpha particles are detected using a detector. This experiment showed that the positive charge of the atom is at the center. Most of the atom’s mass is also confined to a small volume at the center. This region is known as the nucleus of the atom.

## Atomic models

The structure of an atom is explained using various atomic models. Rutherford’s experiment is the basis of Rutherford’s model of an atom. There are also pioneer atomic models like Dalton's model and J.J. Thomson’s model. Let us discuss them briefly.

### John Dalton's model

The idea of the atom was introduced to the scientific community by English scientist John Dalton. He suggested that matter is made up of tiny units called atoms.

According to his atomic model,

• The matter is made up of atoms and an atom is indivisible.
• Atoms of a given element are identical.

### J.J. Thomson’s model

In 1897, J.J. Thomson discovered the electron. He observed that the electrons are ejected from a metal surface. Hence, he concluded that an atom is made up of smaller units or particles. This disproved the fact that the atom is indivisible.

The atomic model developed by J.J. Thomson is called the plum pudding model. According to his atomic model, electrons are embedded in the atom-like plum pudding.

## Experimental setup and procedure

A simple Rutherford’s scattering experiment setup consists of a source of alpha particles. A radioactive element is used as the source of alpha particles, thin gold foil, and a fluorescent screen to detect the scattered radiation. In this experiment, the fast-moving alpha particles are made to strike the thin gold foil. The alpha particles undergo scattering by the gold foil. It was observed that most of the alpha particles were undeflected, they passed the gold foil without changing their path. A small fraction of particles is scattered by a small angle. It was found that about one in 20,000 alpha particles undergoes deflection through an angle of about 90o.

## Rutherford model of an atom

To explain the observations of the scattering experiment, Rutherford came up with the following explanations:

• Most of the alpha particles passed the gold foil without getting undeflected because they did not collide with the gold foil. Hence, it is concluded that the atom mostly consists of empty space.
• Few alpha particles passed close to the nucleus and so, few particles undergo scattering through a small angle.
• Very few particles undergo deflection through a large angle as they collided directly with the nucleus. Hence, it is concluded that the nucleus is positively charged.

According to Rutherford’s atomic model, the atom resembles the solar system. With the nucleus at the center, the electrons revolve around the nucleus just like the planets revolve around the sun. The force between the nucleus and the electron is called electrostatic force. This force is responsible for holding the electron around the nucleus.

The major outcome of this model is as follows:

• Most of the atomic volume is empty. About 99.99% of the volume of the atom is empty.
• The nucleus occupies a very small volume in an atom. The diameter of the nucleus is about 105 times smaller than the diameter of the atom.
• Though the volume of the nucleus is small, much of the mass of the atom is present at the nucleus of the atom.
• Electrons revolve around the nucleus with electrostatic force. Electrostatic force exists between the oppositely charged particles.

## Drawbacks

Rutherford's atomic model has got its drawbacks. They are as follows:

• This model does not explain the stability of the atom.
• In Rutherford’s atomic model, the electrons revolve around the nucleus and it is accelerated. According to maxwell, accelerated charged particles emit electromagnetic radiations. Hence, the revolving electron loses its energy and eventually falls into the nucleus. Thus, Rutherford’s model is not a perfect atomic model.
• This model does not explain the hydrogen spectra. Hydrogen discharge tubes emit radiation of a certain wavelength when high voltage is applied to the tube. Rutherford's atomic model cannot explain this phenomenon. But, this was successfully explained using Bohr’s atomic model.

## Context and Applications

This topic is significant in physics for both undergraduate and graduate courses, especially for

• Bachelors in Electrical Engineering
• Bachelors in Physics
• Masters in Physics

## Practice Problems

Question 1: The nucleus of an atom is ____.

(a) Positive charged

(b) Negatively charged

(c) Neutral

(d) None of the above

Explanation: The nucleus of an atom consists of protons and neutrons. The proton is positively charged and the neutron is neutral. Hence, the net charge of the nucleus is positive.

Question 2: According to Dalton’s model, atoms are___.

(a) Divisible

(b) Charged

(c) Indivisible

(d) None of the above

Explanation: According to Dalton’s model, the atom is indivisible and indestructible. It means that an atom is a single unit that cannot be divided or split into many parts.

Question 3: J.J. Thomson’s model is also known as ____.

(a) Solar system model

(b) Billiard ball model

(c) Orbital model

(d) Plum pudding model

Explanation: J.J. Thomson’s model resembles plum pudding; the electrons are embedded in the atom similar to plum and pudding.

Question 4: In Rutherford’s scattering experiment, the alpha particles undergo deflection due to___.

(a) Neutrons

(b) Electrons

(c) Protons

Explanation: The alpha particles are positively charged particles. Hence, it undergoes scattering due to the repulsion between the proton which is also positively charged particles.

Question 5: Which of the following is not explained by Rutherford’s model?

(a) Stability of the atom.

(b) Scattering of alpha particles

(c) Density of the nucleus

(d) Mass distribution of an atom

Explanation: The important drawback of the Rutherford atomic model is that it does not explain the stability of the atom. As an electron revolves around the nucleus it loses energy and falls into the nucleus. But it is not the case in real hence, Rutherford's model fails to explain atomic stability.

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