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Sir J.J. Thomson’s idea of the atom as a uniformly dispersed element was challenged by Rutherford in 1909. Rutherford conducted the alpha-particle scattering experiment where a radioactive beam was directed towards a gold foil to observe the behaviour of particles. Rutherford realised that the gold atom’s mass should contain a tiny compact nucleus. A small percentage of the alpha particles within the beam were dispersed by huge angles when hitting the gold foil, whereas the majority went entirely through. Rutherford concluded that most of the atom is empty, with most of the mass concentrated in a dense nucleus.
Rutherford’s Atomic Model
The Rutherford model, also known as Rutherford’s atomic model, nuclear atom, or planetary model of the atom, depicts the configuration of atoms presented by Ernest Rutherford, a New Zealand-born, physicist in 1911.
Rutherford’s Atomic Model:
The nucleus of an atom is a dense, positively charged centre of protons.
Atomic mass is solely found in the nucleus.
Atoms have a lot of open space.
Electrons rotate in set orbits around the positively charged nucleus, similar to planets circling the Sun.
Experiment
The nucleus was hypothesised to be tiny and dense to explain the dispersion of alpha particles off thin gold foil found in a set of experiments conducted in 1909 by student Ernest Marsden under the supervision of Rutherford and German scientist Hans Geiger.
A radioactive source generating alpha particles (positively charged particles equal to the helium atom nucleus and 7,000 times heavier than electrons) was surrounded by a lead shield. After entering in via a slit in a lead screen, the radiation was focussed into a narrow beam.
A thin strip of gold foil was put ahead of this slit, and a fluorescent screen covered with zinc sulphide functioned as a metre to detect alpha particles.
When an alpha particle collided with a fluorescent screen, it generated a flash of light known as scintillation that was seen via a viewing microscope mounted on the rear of the screen. The screen panel was moveable, enabling Rutherford and his colleagues to detect if the gold foil was deflecting any alpha particles or not.
Observation
The majority of alpha particles went right through the gold foil, implying that atoms are primarily made up of empty space.
Some alpha particles were slightly deflected, indicating interactions with positively charged particles within the atom.
Some alpha particles were spread at vast degrees, with a few even returning to the origin. Such tremendous repulsion could only be explained by a positively charged and comparatively hefty target particle, such as the suggested nucleus.
The negative electrons which electrically neutralised the positive nuclear charge were thought to move in circular orbits around the nucleus.
The electrostatic force of attraction among electrons and nuclei has been compared to the gravitational force of attraction between the planets and the Sun. The vast majority of this planetary atom was free space, providing a slight obstacle to the passage of the alpha particles.
Rutherford’s atomic model diagram has a tiny centre ball symbolising the nucleus and spherical curves around the nucleus depicting electron paths. The Rutherford model superseded Sir J.J. Thomson’s “plum-pudding” atomic model, wherein a positively charged atom contained electrons resembling plums in a pudding. The Rutherford model, which was entirely built on conventional physics, was quickly supplanted by the Bohr atomic model, which included basic initial quantum theory.
Rutherford’s Atomic Model As A Link Between Thomson’s and Bohr’s Models
Rutherford accepted Thomson’s idea of a spherically shaped atom.
However, he rejected the idea of embedding electrons on a positively charged sphere. Rutherford envisioned a spherical atom with a hefty and tiny nucleus containing protons with electrons circling it. However, it missed a reason for the rotating electron’s fundamental stability without falling into the nucleus, as anticipated by classical physics.
Proceeding with the Rutherford model, Bohr recommended quantisation of the electron’s angular momentum for orbital stabilisation.
Thus, Rutherford’s model acted as a bridge between the Thomson and Bohr models of the atom.
Conclusion
Ernest Rutherford was curious about the configuration of electrons within an atom. So Rutherford created an experiment to investigate this. In this experiment, alpha particles were shot at high speeds and crashed on a thin sheet of gold. The model depicted the atom as a compact, heavy, positively charged centre called a nucleus, in which virtually all of the mass is centred, and surrounding which the light, negative components known as electrons circle at a certain distance, similar to planets rotating around the Sun.
