Rutherford’s Alpha Particle Scattering Experiment

During Rutherford’s alpha-particle scattering experiment, Rutherford and his students used alpha particles to target extremely thin gold foil. A flow of high-energy alpha particles was aimed towards a thin gold foil by a radioactive source. They wrapped a circular fluorescent zinc sulphide screen around the delicate gold foil. A brief flash of light was created whenever alpha particles touched the screen. The particles were supposed to slow down and change orientations only by minor angles. However, some particles deflected, and a few even rebounded. The atomic nucleus was discovered as a result of Rutherford’s experiment. Rutherford’s concept of the atom stated that an atom has a tiny nucleus around which electrons circulate. However, this model could not account for the atom’s stability.

Types of atomic models:

• According to Dalton’s atomic theory, the atom was indivisible and indestructible.
• However, finding two primary particles (electrons and protons) inside the atom caused this portion of Dalton’s atomic theory to collapse.
• It was vital to understand how electrons and protons are organised inside an atom.
• Many scientists developed numerous atomic theories to explain this. J.J. Thomson was the first to offer a model of an atom’s structure. Later, Ernest Rutherford gave the atomic model through his gold foil experiment. Rutherford’s atomic model was followed by Bohr’s atomic model that focussed on explaining the gaps in the previous model.

Thomson’s atomic model:

• Thomson recommended that an atom’s model resemble that of a Christmas pudding. The electrons were similar to currants (dry fruits) around Christmas pudding (sphere of positive charge). 
• According to Thomson, an atom is made up of a positively charged sphere in which the electrons are lodged. The magnitudes of the negative and positive charges are equal. As a result, the atom is electrically neutral as a whole.
• Even though Thomson’s model stated that atoms are electrically neutral, it could not explain the findings of other scientists’ studies.

Rutherford’s atomic model:

Rutherford proposed the nuclear model of an atom based on his experiment, which included the following highlights:

• Each atom has a nucleus, which is a positively charged core. The nucleus contains almost all of an atom’s mass.
• Electrons follow circular pathways all around the nucleus.
• In comparison to the size of the atom, the nucleus is relatively compact.

Bohr’s atomic model:

Neils Bohr proposed the following postulates concerning the model of an atom in an attempt to address the concerns presented against Rutherford’s concept of the atom:

• Only specific unique orbits known as discrete orbits of electrons are permitted inside the atom.
• Electrons do not emit energy when rotating in distinct orbits. Such orbits or shells are referred to as energy levels.

Rutherford’s alpha-particle scattering experiment

Ernest Rutherford wanted to know the arrangement of electrons inside an atom. To understand this, Rutherford devised an experiment. Rapidly travelling alpha (⍺)-particles were launched to drop on a thin sheet of gold in Rutherford’s alpha-particle scattering experiment.

Experiment principle and structure:

• Rutherford used gold foil since he preferred the coating to be as thin as probable. The thickness of such a gold foil was around 1000 atoms.
• ⍺-particles are helium ions with two charges. The quickly moving particles have a significant amount of energy due to their mass of 4amu.
• The subatomic particles in gold atoms were predicted to deflect the alpha particles. He didn’t anticipate detecting substantial deflections because the alpha particles were considerably heavier than the protons.
• Rutherford’s atomic model diagram includes a small central ball representing the nucleus and spherical loops around the nucleus representing the path followed by electrons.

Observations:

The alpha particle scattering experiment, on the other hand, yielded completely unexpected findings. Following are some of the observations made:

• The majority of the fast-moving alpha particles went through the gold foil undisturbed.
• The foil redirected a few of the alpha particles at small angles.
• Strangely, one particle out of every 12000 seemed to rebound.

“This outcome was nearly as unbelievable as firing a 15-inch shell at a piece of tissue paper, and it rebounds to hit you,” Rutherford said.

The conclusion from the experiment findings

Since the majority of the alpha particles travelled across the gold foil without being deflected, Rutherford reasoned from the alpha-particle scattering experiment that:

• Almost all of the space within the atom is vacant.
• Only a few particles were diverted from their intended route, implying that the atom’s positive charge takes up a relatively small space.
• By 180 degrees, only a small percentage of alpha particles were deflected, implying that the gold atom’s positive charge and mass were contained in a relatively limited volume inside the atom.

He also computed that the radius of the nucleus is around 105 times smaller than the radius of the atom based on the data.

Conclusion

John Dalton presented the first atomic theory in 1808, which saw the atom as the fundamental, indivisible element of substance. Later experiments demonstrated that atoms are divisible and comprise three primary particles: electrons, protons, and neutrons. Following the discovery of subatomic particles, numerous atomic models were proposed to describe the structure of the atom. Rutherford reasoned that an atom is composed of a small positively charged nucleus at its centre, with electrons circling in circular orbits around it. The Rutherford model was an advance over the Thomson model, but it did not explain the atom’s stability or how the electron somehow doesn’t drop into the nucleu