Thomson’s model of the atom: It was proposed by J. J. Thomson in 1898. According to this model, the positive charge of the atom is uniformly distributed throughout the volume of the atom and the negatively charged electrons are embedded in it like seeds in a watermelon.
Rutherford’s α-scattering Experiment
Rutherford and his two associates, Geiger and Marsden, study the scattering of the α-particles from a thin gold foil in order to investigate the structure of the atom.
Rutherford’s observations and results:
– Most of the α-particles pass through the gold foil without any deflection. This shows that most of the space in an atom is empty.
– Few a-particles got scattered, deflecting at various angles from 0 to p. This shows that the atom has a small positively charged core called ‘nucleus’ at the centre of the atom, which deflects the positively charged α-particles at different angles depending on their distance from centre of nucleus.
– Very few α-particles (1 in 8000) suffer deflection of 180°. This shows that the size of the nucleus is very small, nearly 1/8000 times the size of the atom.
This graph shows the deflection of a number of particles with an angle of deflection q.
Rutherford’s nuclear model of the atom:
According to this the entire positive charge and most of the mass of the atom is concentrated in a small volume known as the nucleus with electrons revolving around it just as planets revolve around the sun.
Bohr’s model: Bohr combined classical and early quantum concepts and gave his theory of hydrogen and hydrogen-like atoms which have only one orbital electron. His postulates are
An electron can revolve around the nucleus only in certain allowed circular orbits of definite energy and in these orbits, it does not radiate. These orbits are known as stationary orbits.
Spectral series of the hydrogen atom: When the electron in an H-atom jumps from higher energy level to lower energy level, the difference of energies of the two energy levels are emitted as radiation of particular wavelength, known as a spectral line. Spectral lines of different wavelengths are obtained for the transition of electron between two different energy levels, which are found to fall in a number of spectral series given by
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