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Ernest Rutherford, a British scientist who won the Nobel prize in chemistry in 1908, conducted the alpha particle scattering experiment. He gave the Rutherford Atomic Model based on his observation and proposed the atomic structure of elements. He was interested in knowing how the electrons are placed in an atom. Still, the 𝛂-particle scattering experiment gave an unexpected result that led to the discovery of the atom’s nucleus. The major components of the alpha scattering experiment were gold foil (100 nm thickness), zinc sulfide screen, and fast-moving alpha particles. In his experiment, he directed high-energy streams of alpha particles on a thin sheet of gold foil to study the trajectory of these particles, and he placed a fluorescent zinc sulfide screen around the delicate gold foil To study the deflection caused to the α-particles. The alpha Particle scattering experiments indicate an atom’s structure.
History
J.J. Thomson gave a model in which he proposed that an atom is similar to a plum pudding in which a positive charge is spread all over the model, and a negative charge is embedded in it like dry fruits. The number of positive and negative charges is the same. Hence, the atom is neutral. However, this model explained that atoms are electrically neutral but failed to explain specific experimental results about the atomic structure of elements. Ernest Rutherford was interested in knowing how electrons are placed in an atom, and hence designed this experiment to indicate the atom’s structure.
The Alpha Particle Scattering Experiment
The major components of the alpha scattering experiment were:
Gold foil: A thin sheet of gold foil (100 nm) was selected as it has the thinnest layer of about 1000 atoms.
𝛂-particles: they are doubly-charged helium ions with considerably high energy and a mass of 4u, which is more than the protons of a gold atom.
Zinc sulfide screen: a fluorescent sheet of zinc sulfide was placed around the foil to study the deflections.
𝛂-particle emitter: generates a high energy beam of 𝛂-particles.
In this experiment, Rutherford bombarded a thin sheet of gold foil with high-energy streams of fast-moving 𝛂-particles from the emitter. He expected that 𝛂-particles would be deflected by small angles only as the 𝛂-particles were much heavier (4 u mass) than the protons of a gold atom, but it gave unexpected results. He saw that:
Small angles deflected some particles.
Most particles passed straight(undeflected) through the gold foil
Few particles are deflected by large angles, and some even rebound(1 out of 12000 particles)
He gave the Rutherford Atomic Model from these results, which contradicted the Thomson Atomic Model. Rutherford’s alpha-particle scattering experiment explained the atomic structure of an atom.
Observations of the experiment
The following observations were made from the alpha particle scattering experiment:
Most of the space inside an atom is empty: As a significant fraction of particles bombarded on the gold foil passed straight through the foil, it was concluded that a lot of space inside an atom is empty.
Distribution of positive charge is non-uniform: Some 𝛂-particles were deflected by very small angles; hence, it was assumed that positive charge occupies little space and is non-uniformly distributed.
All the positive charge is concentrated in a small volume: Few 𝛂-particles, 1 out of 12000, were deflected by an angle of 180 degrees showing that all the positive charge is concentrated in a very small volume compared to the total volume of the gold atom.
Rutherford Atomic Model
From the observations, he proposed Rutherford’s atomic model, which indicated the atomic structure of elements.
Rutherford put forward the nuclear model based on the experiment, which states:
There is a positively charged center of the atom called the nucleus, where all the atom’s positive charge is concentrated. It is very small and around 105 times less than the atom’s radius. All the mass of the atom is concentrated here.
The negatively charged particles surround the atom and revolve at a very high speed in fixed circular paths called orbits.
The positively charged particles (protons) and negatively charged particles (electrons) are held together by a powerful electrostatic force of attraction.
Drawbacks of The Rutherford Atomic Model:
There are some limitations of the Rutherford Atomic Model as it failed to explain certain things:
According to Rutherford, electrons move in circular orbits around the nucleus. We know by Maxwell’s theory that any charged particle undergoing circular motion will experience acceleration and emit electromagnetic radiation. This would lead to a shrinking orbit as the revolving electron will lose the energy and collapse to fall into the nucleus.
Rutherford could not explain the stability of atoms, as it highly contradicts the Maxwell theory. According to the calculations, the electron will collapse in less than 10-8 seconds, making the electron highly unstable.
Rutherford did not mention anything about the arrangement of electrons in the orbits, which is also one drawback.
Conclusion
Rutherford’s alpha-particle scattering experiment explained a lot of theories that the Thomson model failed to. In the experiment, significant components were gold foil, alpha particles, and zinc sulfide screen on which deflection of atoms was studied. 𝛂-particles were made to fall on the gold foil, and the trajectory of these particles after interaction with the foil was observed to give The Rutherford Atomic Model. The alpha particle scattering experiment was able to indicate the atom’s structure, and it led to the discovery of the nucleus, a densely concentrated positively charged center of the atom. It also stated that there is a lot of space inside the atom, and the electrons revolve around atoms in orbits. Although it explained a lot of theories, it failed to explain the stability of atoms, as it contradicted Maxwell’s theory. Rutherford’s model did not explain the arrangement of electrons in orbit as well; hence, this model has certain limitations.
