Rutherford Gold Foil or Alpha Particles Scattering Experiment

Ernest Rutherford conducted the gold foil experiment or the alpha particles scattering experiment in the year 1911. Based on the results, Rutherford presented his atomic model, which laid the foundation of modern-day’s atomic theory.

Rutherford’s Gold Foil Experiment or the Alpha Particles Scattering Experiment

In this experiment, he bombarded a very thin gold foil with alpha particles to find the arrangement of electrons and positively charged particles in an atom. His experiment led to the discovery of a small, highly dense, and positively charged mass at the center of an atom. This positively charged mass was later known as the nucleus. Although people around the world give credit for the discovery of the nucleus to Ernest Rutherford, he never mentioned the word nucleus in his research paper.

Ernest Rutherford (30 August 1871 – 19 October 1937)

Lord Ernest Rutherford, the one who discovered the nucleus and protons

Ernest Rutherford was a New Zealand-born British physicist. His full name was Ernest, Baron Rutherford of Nelson. Due to his immense contribution to the study of radioactivity and atoms, he is also known as the father of nuclear physics. Rutherford won the Nobel Prize for Chemistry in 1908 and served as president of the Royal Society for the period of nearly five years from the year 1925 to 1930. The most famous work of Rutherford was when he overturned the plum-pudding atomic model of J. J. Thomson based on the results of the gold foil or the alpha particles scattering experiment.

Rutherford Gold Foil or Alpha Particles Scattering Experiment

Alpha particles are positively charged particles comprising two numbers of protons and two numbers of neutrons. In other words, they are helium atoms without two numbers of electrons. Some atoms of heavy elements with a large neutron-to-proton ratio in their nuclei emit alpha particles to bring parent nuclei to a more stable configuration.

Rutherford discovered the nucleus in an experiment widely known as the gold foil experiment. In his experiment, Rutherford surrounded a thin sheet or foil of gold (0.00004 cm thickness) with a screen made of Zinc Sulfide. The screen allowed passage for a thin beam of alpha particles emitted from the radioactive source. The radioactive source used by Rutherford for emitting alpha particles was Radon. He bombarded this thin foil of gold with alpha particles in an evacuated chamber. Alpha particles after reflecting or passing through the gold sheet struck the Zinc Sulfide screen and produced a red glow at the point of contact.

Observations of the Gold Foil or the Alpha Particles Scattering Experiment

Rutherford noted locations where alpha particles struck the screen and observed as follows:

Gold Foil bombarded with Alpha Particles during the Experiment

1. Most of the alpha particles passed straight through the foil without any deflection from their path.
2. A small fraction of them was deflected from their original path by small angles.
3. Only a few particles bounced back.

Step-by-step demonstration of Alpha particles scattering experiment. Video courtesy blenderIITB

Conclusions of the Gold Foil or the Alpha Particles Scattering Experiment

Based on the above observations made during the alpha particles scattering experiment, Rutherford made the following conclusions:

1. Most of the space in an atom was empty because alpha particles went straight.
2. The deflection of a small fraction of alpha particles occurs due to the presence of a heavy positively charged mass in the atom.
3. The positively charged mass is very small and centrally located because only a few particles bounced back. He named this positively charged mass discovered by him – the nucleus of an atom.

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Rutherford Atomic Model

Based on his gold foil experiment, Rutherford suggested an atomic model. According to this model, an atom consists of mainly two parts:

Rutherford Atomic Model and its postulates

Part-1: The centrally located nucleus

1. The nucleus is a centrally located positively charged mass.
2. As electrons have negligible mass, thus, the entire mass of the atom is present inside the nucleus. It is the densest part of the atom.
3. If compared to the size of the atom as a whole the size of the nucleus is very small.

If we consider a circular stadium as an atom, then its nucleus is no more than a cricket ball placed at the center of the stadium.

Part-2: The outer circular orbits

1. Electrons revolve around in circular orbits (shells) in the space available around the nucleus.
2. An atom is electrically neutral i.e., the number of protons and the number of electrons present in an atom is equal.

Thus, a model proposed by Rutherford is somewhat similar to that of the solar system. Just as in the solar system, the sun is at the center and the planets revolve around it, in an atom the electrons revolve around the centrally located nucleus containing protons. Therefore, some people also consider Rutherford’s atomic model as a planetary model of an atom. However, it is not correct, Niels Bohr, a Danish physicist presented the actual planetary model in the year 1915.

Rutherford discovered proton in the year 1917, much later after the discovery of the nucleus. He discovered that positively charged particles present in the nucleus are similar to those of H⁺ ions and named them ‘protons’.

Frequently Asked Questions

Q1. Why did Rutherford use a gold foil in this experiment?

Ans. Rutherford used gold foil in his experiment because of all metals known to mankind, gold is the most malleable. As he wanted his foil to be as thin as possible, therefore, to achieve the same he used gold.

Q2. What is the thickness of the gold foil used by Rutherford?

Ans. The gold foil used by Rutherford in his experiment was about 4 micrometers i.e. 2410 atoms thick. Please note the diameter of a single gold atom is 1.44 x 10^-10 m.

Q3. Why did Rutherford use alpha particles in his experiment?

Ans. Some of the important reasons for the use of alpha particles by Rutherford in the gold foil experiment are:

1. Rutherford discovered alpha particles in the year 1899.
2. Alpha particles are easily available. Heavy elements such as uranium, radium, polonium, and radon emit alpha particles during radioactive decay.
3. Detection of alpha particles was easy. They produce a flash of light on striking a screen coated with zinc sulfide.
4. Alpha particles are helium atoms without electrons. Therefore, they are doubly charged particles of mass 4 amu. The alpha particles move at a very high speed with a considerable amount of energy, which is sufficient to penetrate through the thin gold foil used in the experiment. 
5. The health effects of direct exposure to alpha particles are not a major concern. Although alpha particles are very energetic, they are so heavy that they are unable to travel very far from the atom. They lack the energy to penetrate even the outer layer of human skin.