What is Rutherford Atomic Model?
In 1904, J.J. Thomson proposed the plum pudding model of an atom. He stated that an atom is a positive charge sphere in which electrons (negative charge) are embedded.
He also stated that the atom is electrically neutral as positive charge and negative charge present inside the nucleus of an atom are equal. But this model could not describe the stability of the atom.
Ernest Rutherford, a British scientist conducted an experiment named ‘alpha gold-foil, and thus based on those observations, he proposed the atomic structure of elements and which came to known as the Rutherford Atomic Model.
Rutherford Atomic Model and His Alpha Scattering Experiment
Rutherford performed an experiment by bombarding a thin sheet of gold with alpha particles and then studied the path of these particles after they contacted with the gold foil.
Rutherford, in his experiment, bombarded high-energy streams of α-particles from a source of radioactive element of a thin sheet (100 nm thickness) of gold.
He placed a fluorescent zinc sulfide screen around the thin gold foil to study the variation in path of alpha particles.
Rutherford’s observations thus opposed Thomson’s atomic model. Reason for using gold foil in the experiment Gold is the most malleable metal of all metals.
Thus, It can easily be converted into very thin sheets. Reason for choosing alpha particles in the experiment Alpha particles are referred to as double-charged Helium atoms, and they have more momentum. Furthermore, they deviate least from their path.
Alpha Scattering Experiment's Observation
The observations made by Rutherford are mentioned below:
1. A greater fraction of the α-particles that were bombarded on the gold sheet passed through it without any deflection, and hence that means that most of the space in an atom is empty.
2. Some of the α-particles were deflected by the gold sheet at minor angles, and it was found that the positive charge in the atom is not distributed uniformly.
3. Very few of the α-particles were commonly deflected back, that is only a few α-particles one in every 12000 alpha particles bombarded were deflected at an angle 180o. So, the volume occupied by the positively charged particles in an atom inhabits a small volume as compared to the total volume of an atom.
Rutherford Atomic Model
Based on the above finding and conclusions, Rutherford further proposed the atomic structure of elements. Hence, the Rutherford’s atomic model proposed that:
1. The positively charged particles and most of the mass of an atom was concentrated in a very small volume inside an atom. This small positively charged body present in the atom is named as nucleus.
2. He also proposed that the negatively charged electrons surround the nucleus present in an atom. He demonstrated that the electrons that surrounds the nucleus revolve around it at a very high speed in circular paths. These circular paths are termed as orbits.
3. He stated that the electrons having negatively charged and the nucleus having the positively charged constituents are generally held together by a strong electrostatic force of attraction.
Rutherford Atomic Model and Modern Science
Rutherford’s finding helped the scientists to realise that the atom is not just made up of a single particle, but instead it consists of smaller subatomic particles.
Rutherford’s gold foil experiment also helped the scientists to find the exact atomic structure of an atom. Scientists eventually discovered that atoms have a positively charged nucleus present in the centre and radius of atom found to be 1.2 × 10−15 meters × [atomic mass number]1⁄3.
Subsequently, scientists found the number of electrons present in an atom by using X-rays. When an X-ray is passed through an atom, some part of it usually scatter, while the remaining rays pass through the atom.
As the X-ray commonly loses its intensity when electrons are scattered, then the number of electrons contained in an atom can be accurately estimated thus by finding the rate of decrease.
Limitation of Rutherford Atomic Model
The Rutherford atomic model was based on experimental observations, yet it failed to explain few things as mentioned below;
• Rutherford’s model was unable to explain the stability of an atom.
• Rutherford model did not discuss anything about the arrangement of an electron in orbit as well.
• According to Rutherford’s atomic model, electrons revolve around the nucleus in a circular path called orbits. But particles that are in motion around the nucleus on a circular path would experience acceleration, and this acceleration causes radiation of energy by charged particles. Ultimately, the electrons should lose energy and fall into the nucleus of an atom.
• Other drawback of the Rutherford model was a that he was unable to explain anything about the distribution of electrons in an atom.
• Definite lines in the hydrogen spectrum could also not be clarified by this model of atom.
Rutherford Atomic Model Citations
- Simulation of Rutherford backscattering spectrometry from arbitrary atom structures. Phys Rev E . 2016 Oct;94(4-1):043319.
- Comparison of Rutherford’s atomic model with the Standard Model of particle physics and other models.
- Rutherford’s Contribution. Science 25 Apr 1997: Vol. 276, Issue 5312, pp. 513-517