Ernest Rutherford, a New Zealand born physicist, discovered the nucleus of the atom as a small, dense and positively charged atomic core. According to him, there was a small positively charged centre surrounded by circulating electrons in orbits. This discovery was made through a series of experiments called Geiger- Marsden or the Gold Foil Experiment. But there were some drawbacks to Rutherford’s atomic model.
These drawbacks of Rutherford’s Atomic Model led to significant discoveries in the future.
What was the Atomic Model Presented by Rutherford?
Thousands of years back, before Rutherford described the structure of an atom, ancient Greek Philosopher Democritus, around 450 BC, spoke of the atom as the smallest indivisible particle. He formulated the word “Atomos”, which meant ‘uncut’. Later, many philosophers and scientists like Galileo, Newton, Boyle, and Lavoisier explored the idea of an atom. In 1808, British Chemist John Dalton, the father of modern atomic theory, said that matter was made of atoms.
In 1897 British Physicist J.J Thompson discovered the negatively charged electrons within an atom and proposed the Plum Pudding Model of Atoms. But this model failed to explain the atomic structure of elements.
Earnest Rutherford, assisted by Hans Geiger and Ernest Marsden, conducted the Gold Foil Experiment in 1911, which helped him postulate the Rutherford Atomic Model. A thin sheet of gold foil of 100 nm thickness was bombarded with alpha particles from a radioactive source. A screen made of Zinc Sulphide was placed around the gold foil to study the extent of the deflection of the alpha particles when they hit the Zinc Sulphide screen. The trajectory of the alpha particles was then carefully observed by observing the scintillations they produced on the screen.
Rutherford experiment resulted in the following observations:
- Most of the alpha particles traversed through the gold foil, and hence Rutherford assumed that most of the space in an atom is empty.
- Some alpha particles deflected slightly at minute angles. This suggested interactions with the other positively charged particles in the atom.
- Very few were scattered at large angles.
- Few alpha particles deflected at 180 degrees. Rutherford concluded that the positive charge in an atom is not uniformly distributed and is concentrated in minimal volumes.
- Some alpha particles bounced back, and Rutherford later said, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back and hit you.”
Limitations of Rutherford’s Atomic Model:
The following drawbacks to the Rutherford Model could be observed:
- According to him, an atom’s entire mass and positive charge were concentrated in a tiny region at the centre known as the nucleus.
- Protons produced a positive charge on the nucleus. The Magnitude of the positive charge on the nucleus varied for different elements as the number of protons differed in each element.
- Negatively charged electrons surrounding the nucleus, and the number of electrons was equal to protons in the nucleus.
- The electrons circulate the nucleus at high speed like planets revolving around the sun. They travelled in circular orbits.
- There was an electrostatic force of attraction between the electrons and the nucleus of an atom, which was similar to the gravitational force of attraction between the revolving planets and the sun.
- Rutherford believed that most of the space in an atom was empty.
- According to Rutherford, the atom was neutral as electrons are negatively charged, and the dense small nucleus is positive in charge.
Drawbacks of Rutherford’s Model:
- Rutherford failed to explain the stability of the atoms. According to him, the electrons revolve around the positively charged nucleus in a circular fixed orbit. If then, the electrons would accelerate and resultantly release electromagnetic radiation in motion. After that, revolving electrons will lose energy; the orbits will shrink gradually, and electrons will collapse in the nucleus of an atom.
- The model also failed to explain the arrangement of electrons in orbit.
- The model also failed to explain the existence of definite lines in the hydrogen spectrum.
Conclusion
In this way, the model acted as a catalyst for more remarkable changes to be observed in the patterns of the atom and the way its protons and electrons behaved. This ultimately led to improved experiments and models where such patterns could be correctly identified.
