Bohr's Model of an Atom:

Bohr's model of an atom According to Rutherford"s model, an atom consists of a positive nucleus with the electrons moving around it in circular orbits. However it had been shown by J. C. Maxwell that whenever an electron is subjected to acceleration, it emits radiation and loses energy. As a result of this, its orbit should become smaller and smaller and finally it should drop into the nucleus by following a spiral path. This means that atom would collapse and thus Rutherford"s model failed to explain stability of atoms. To overcome the defect of Rutherford"s model, Niels Bohr in 1913 gave a modification based on Quantum theory of radiation. The important postulates are:

• The electrons revolve round the nucleus only in certain selected circular paths called orbits. These orbits are associated with definite energies and are called energy shells or energy levels or quantum levels. These are numbered as 1, 2, 3, 4 ....... etc. (starting from the nucleus) are designated as K, L, M, N ......etc..
• As long as an electron remains in a particular orbit, it does not lose or gain energy. This means that energy of an electron in a particular path remains constant. Therefore, these orbits are also called stationary states.
• Only those orbits are permitted in which angular momentum of the electron is the whole number multiple of h/2π, where "h" is planck's constant. An electron moving in a circular orbit has an angular momentum equal to mnr where m is the mass of the electron and n, the angular momentum, mnr is a whole number multiple of h/2π. In other words, angular velocity of electrons in an atom is quantised.
• If an electron jumps from one stationary state to another, it will absorb or emit radiation of a definite frequency giving a spectral line of that frequency which depends upon the initial and final levels. When an electron jumps back to the lower energy level, it radiates same amount of energy in the form of radiation.