ernest rutherford, father of nuclear physics

Early Models of Atom Were Highly Useful to Physics


Thomson’s Model of the Atom

The first proposal regarding the possible arrangement of positively and negatively charged particles in the atom was made by J. J. Thomson in 1907.

The first proposal regarding the possible arrangement of positively and negatively charged particles in the atom was made by J. J. Thomson in 1907. This is Thomson’s model of the atom.

In this model, the entire positive electric charge of the atom is distributed within the sphere of atomic radius which is in the order of 10-10m. The negatively charged electrons are embedded in this sphere of positive charge at definite points like a plum in the pudding. The electrons are thought to oscillate with definite frequencies about their equilibrium positions.

In this model, the entire positive electric charge of the atom is distributed within the sphere of atomic radius which is in the order of 10-10m. The negatively charged electrons are embedded in this sphere of positive charge at definite points like a plum in the pudding. The electrons are thought to oscillate with definite frequencies about their equilibrium positions.

It is well known that when light emitted by different elements is analyzed spectroscopically, the emission spectrum is found to consist of light of definite wavelengths. These different wavelength components appear as spectral lines of definite frequencies or wavelengths in the spectrogram.

According to the electromagnetic theory of light, when an electron oscillates with a definite frequency, it emits light of a particular frequency. In addition, light having frequencies twice, thrice, etc of the oscillation can also be emitted. This, of course, will take place with less intensity. In the hydrogen atom, there is only one electron, which will oscillate within a particular frequency.

So, according to Thomson’s model, the emission spectrum of hydrogen will mainly consist of light of this particular frequency. However, in this case, the emission spectrum is found to be made of a large number of spectral lines of different frequencies having comparable intensities.

The frequencies of these lines do not have any correspondence with the expected oscillation frequency of the electron in this case.


NUCLEAR PHYSICS GLOSSARY: DEFINITIONS, TERMS, MEANING, EXPLANATION


Rutherford’s Model of the Atom

ernest rutherford atom nucleus model experiment

In this model, the atom consists of a central positively charged nucleus and is surrounded by negatively charged electrons. The central nucleus has a positive charge which is different in magnitude for different elements.

In neutral atoms, the number of electrons outside the nucleus is equal to the number of positive charges in the nucleus. The mass of the atom is concentrated entirely in the nucleus.

The volume of the nucleus is much smaller than the volume of the atom by the ratio of 1:1013. In order to account for the stability of the atom, electrons are revolving around the nucleus in closed orbits. Thus centrifugal force balances the force of attraction and keeps them in their path.

It is known from the electromagnetic theory of light, that an accelerated or decelerated electrically charged particle emits electromagnetic radiation and thereby loses its energy so its velocity is decreased.

Since the electron revolving in its orbit is acted upon by the centripetal force of attraction, its motion is accelerated. Thus it will lose energy by the emission of electromagnetic radiation. So due to the attractive force of the nucleus, it will move in orbits of continually decreasing radii and its path will be a spiral.

Finally, it will fall into the nucleus itself and will disappear. So, in this model, there could not be any atom with the positively and negatively charged parts existing separately in it, which is contrary to the observed facts.


Next see Bohr’s Theory of Atom: Postulates, Theory, Advantages, Drawbacks, Legacy, Neils Bohr, Etc.


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