(a) Every material is composed of minute particles known as atom. Atom is indivisible i.e. it cannot be subdivided. It can neither be created nor be destroyed.
(b) All atoms of same element are identical physically as well as chemically, whereas atoms of different elements are different in properties.
(c) The atoms of different elements are made up of hydrogen atoms. (The radius of the heaviest atom is about 10 times that of hydrogen atom and its mass is about 250 times that of hydrogen.)
(d)The atom is stable and electrically neutral.
Thomson’s Atom Model :
(a) The atom as a whole is electrically neutral because the positive charge present on the atom (sphere) is equal to the negative charge of electrons present in the sphere.
(b) Atom is positively charged sphere of radius 10–10 m in which electron are embedded in between.
(c) The positive charge and the whole mass of the atom is uniformly distributed throughout the sphere
Shortcomings of Thomson’s Model :
(i) The spectrum of atoms cannot be explained with the help of this model.
(ii) Scattering of α-particles cannot be explained with the help of this model.
Rutherford Atom Model :
Rutherford performed experiment of scattering of α-particles by thin gold foil and concluded:
(a) Most of the a-particles went straight through the gold foil and produced flashes on the screen as if there were nothing inside gold foil. Thus the atom is hollow.
(b) Few particles collided with the atoms of the foil which have scattered or deflected through considerable large angles. Few particles even turned back towards source itself.
(c) The entire positive charge and almost whole mass of the atom is concentrated in small centre called nucleus.
(d) The electrons could not deflected the path of a a-particles i.e. electrons are very light.
(e) Electrons revolve round the nucleus in circular orbits.
So, Rutherford 1911, proposed a new type of model of the atom. According to this model, the positive charge of the atom, instead of being uniformly distributed throughout a sphere of atomic dimension is concentrated is a very small volume (less than 10–13 m in diameter) at its centre. The central core, now called nucleus, is surrounded by clouds of electron makes. The entire atom electrically neutral.
The scattering angle decreases with increasing impact parameter.
Bohr’s Theory of Hydrogen Atom :
Bohr’s theory of hydrogen atom is based on the following assumption :
- Only these orbits are possible for which the orbital angular momentum of the electron is equal to an integral multiple of h/2π ; where h is Plank constant.
- The electron moving in such allowed orbits does not radiate electromagnetic radiations. Thus the total energy of the electron revolving in any of the so many stationary orbits remains constant.
- Electromagnetic radiations are emitted if an electron jumps from stationary orbit of higher energy E2 to another stationary orbit of lower energy E1. The frequency n of the emitted radiation is related by the equation.
Defects of Bohr Model:
(a) This model could not explain the fine structure of spectral lines, Zeeman effect and Stark effect.
(b) This model is valid only for single electron systems.
(c) This model is based on circular orbits of electrons whereas in reality the orbits are elliptical.
(d) Electron is presumed to revolve round the nucleus only whereas in reality it also rotates about its own axis.
(e) This model could not explain the quantization condition of angular momentum (i.e. the classical and quantum theories were used simultaneously).
(f) This model could not explain the intensity of spectral lines.
(g) It could not explain the doublets obtained in the spectra of some of the atoms.