Rutherford’s Atomic Model – The plum pudding model is given by J. J. Thomson failed to explain specific experimental results associated with the atomic structure of elements.
Ernest Rutherford, a British scientist, conducted an experiment and based on the observations of this experiment, he proposed the atomic structure of elements and gave Rutherford Atomic Model.
The concept of atom dates back to 400 BCE, when Greek philosopher Democritus first conceived the idea. However, it was not until 1803 John Dalton proposed the idea of the atom again. But at that point in time, atoms were considered indivisible.
This idea of an atom as indivisible particles continued until 1897, when British Physicist J.J. Thomson discovered negatively charged particles which were later named electrons.
Rutherford’s Atomic Model Experiment
In Rutherford’s experiment, he bombarded high energy streams of α-particles on a thin gold foil of 100 nm thickness. The streams of α-particles were directed from a radioactive source.
He conducted the experiment to study the deflection produced in the trajectory of α-particles after interaction with the thin sheet of gold. To study the deflection, he placed a screen made up of zinc sulphide around the gold foil.
Observations of Rutherford Model Experiment
Based on the observations made during the experiment, Rutherford concluded that
1. Major space in an atom is empty – A large fraction of α-particles passed through the gold sheet without getting deflected. Therefore, the major part of an atom must be empty.
2. The positive charge in an atom is not distributed uniformly, and it is concentrated in a very small volume – Few α-particles, when bombarded, were deflected by the gold sheet. They were deflected minutely and at very small angles. Therefore he made the above conclusion.
3. Very few α-particles had deflected at large angles or deflected back. Moreover, very few particles had deflected at 180o. Therefore, he concluded that the positively charged particles covered a small volume of an atom in comparison to the total volume of an atom.
Rutherford’s Atomic Model
Based on the above observations and conclusions, Rutherford proposed the atomic structure of elements. According to the Rutherford atomic model:
1. The positively charged particles and most of the mass of an atom was concentrated in an extremely small volume. He called this region of the atom a nucleus.
2. Rutherford model proposed that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it at very high speed in circular paths. He named these circular paths orbits.
3. Electrons being negatively charged and nucleus being a densely concentrated mass of positively charged particles are held together by a strong electrostatic force of attraction.
Rutherford needed to develop an entirely new model of the atom to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space.
In contrast, the highly deflected particles must have experienced a tremendously powerful force within the atom. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom’s interior, which he called the nucleus.
Rutherford’s atomic model became known as the nuclear model. In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the centre of the atom.
The electrons are distributed around the nucleus and occupy most of the volume of the atom. It is worth emphasizing just how small the nucleus is compared to the rest of the atom. If we could blow up an atom to be the size of a large professional football stadium, the nucleus would be about the size of a marble.
Rutherford’s model proved to be an important step towards a full understanding of the atom. However, it did not completely address the nature of the electrons and the way in which they occupied the vast space around the nucleus. It was not until some years later that a full understanding of the electron was achieved. This proved to be the key to understanding the chemical properties of elements.
Limitations of Rutherford’s Atomic Model
Rutherford’s experiment was unable to explain certain things. They are:
- Rutherford’s model was unable to explain the stability of an atom. According to Rutherford’s postulate, electrons revolve at a very high speed around a nucleus of an atom in a fixed orbit. However, Maxwell explained accelerated charged particles release electromagnetic radiations. Therefore, electrons revolving around the nucleus will release electromagnetic radiation.
- The electromagnetic radiation will have energy from the electronic motion, as a result of which the orbits will gradually shrink. Finally, the orbits will shrink and collapse in the nucleus of an atom. According to the calculations, if Maxwell’s explanation is followed, Rutherford’s model will collapse with 10-8 seconds. Therefore, the Rutherford atomic model did not follow Maxwell’s theory and was unable to explain an atom’s stability.
- Rutherford’s theory was incomplete because it did not mention anything about the arrangement of electrons in orbit. This was one of the major drawbacks of the Rutherford atomic model.