Before the introduction of Rutherford’s atomic model, scientists used to think that an atom is the smallest unit of all the matter. After some discovery and further studies, it was noticed that atom consists of protons, electrons and neutrons. Electrons revolve around the nucleus which is the centre of an atom and consists of protons and neutrons, in the same way as planets revolve around the sun in the solar system. The scientist who proposed this concept was Ernest Rutherford. He did an experiment known as the gold foil experiment to understand the composition of an atom.
Gold foil experiment:
This experiment was done to understand the structure of an atom. He did this experiment to verify the plum pudding model given by J.J Thomson, but he ended the experiment with some new conclusions. In this experiment he used alpha particles and gold foil. Alpha particle is the helium nucleus having two protons and two neutrons. He bombarded positive alpha particles on the gold foil of thickness approx 8.6 x 10-6 cm and took the observation on a screen of zinc sulphide which was kept behind the gold foil. He observed that some beams were deflected back, some of them passed straight and some bend to small angles. These observations led Rutherford to conclude the following points:
- Most of the space in an atom is vacant.
- There is a nucleus at the center which is positively charged and electrons revolve around it in circular orbits.
The second point contradicted the theory proposed by James Maxwell about the charged particles. He said that an accelerated charged particle would lose energy in the form of radiations. So if electrons will continuously move in a circular orbit around the nucleus, then it will lose energy and after some time it would fall on the nucleus. It would take around 10-8 seconds for an electron to lose all its energy and collapse in the nucleus. So the theory given by Rutherford could not explain the stability of an atom.
Thus Rutherford model failed to explain the structure of an atom in its entirety. Later on another model came into existence but before that we should know about the electromagnetic theory.
James Maxwell proposed an electromagnetic theory. As we move on to electromagnetic theory, let us first understand the term electromagnetic waves. In 18th century Young performed an experiment known as the Young’s double slit experiment in which he observed that light behaves like a wave, but James Maxwell modified it and said that light is an electromagnetic wave.
Electromagnetic waves are defined as the waves that can travel through vacuum. He said that when a charged particle is accelerated, it can move through the electric and the magnetic fields around it. These fields move in the form of waves hence are known as the electromagnetic waves.
Let us understand the various terms associated with the study of waves:
Crest: The highest point of any wave.
Trough: The lowest point of any wave.
Wavelength: The distance between two consecutive troughs and crests.
Amplitude: Height of the wave from the mean position.
Frequency: The number of waves or the number of troughs that pass through a point in a given period of time. Unit-: second-1 or Hz.
Characteristics of Electromagnetic Waves:
- Both the magnetic and electrical fields are mutually perpendicular to the direction of propagation of electromagnetic wave and to each other.
- Do not require a medium
- Travel with the velocity equal to the speed of light.
Varying the wavelength and frequency, keeping the velocity of light constant we get a large spectrum known as the electromagnetic spectrum.
To know more about the different theories associated with the structure of an atom or detailed version of Rutherford’s model and more about the electromagnetic spectrum, please watch the above video.