Bohr’s Atomic Model:
(Bohr’s atomic model for hydrogen atom)
Neils Bohr’s, 1913 proposed a model of an atom based on the planks quantum theory of radiation.
Main Postulates of Bohr’s theory:
The basic postulates of Bohr’s theory are:
- An atom consists of small, heavily positively charged nucleus around which electrons revolve in definite circular paths called orbits.
- These orbits are associated with definite energies called energy shells or energy levels. They are designated as K, L, M, N, O…..etc. shells or numbered as 1,2,3,4,5,…. etc. from the nucleus.
- As long as the electrons remain in a particular orbit/energy shell their energy remains constant. This accounts for the stability of an atom.
- Only those orbits are permitted in which the angular momentum of the electron is a whole number multiple of h/2π where h is Planck’s constant. Any moving body taking a circular orbit has an angular momentum equal to the product of its mass (m), velocity (v) and radius of orbit (r).
In other words the angular momentum of an electron
mvr = n h/2π
Where n= 1, 2, 3…
5. Electrons can either lose or absorb energy abruptly when they jump from one energy level to another. For instance, when an electron moves from normal or ground state E1 of an atom i.e. the state of lowest energy as required by its ‘n’ and ‘l’ values, to a higher level, it causes the atom to be in its excited state E2 i.e. where electrons of an atom occupy energy levels higher than those permitted by its ‘n’ and ‘l’ values. The reverse is also true and the change in energy is:
Bohr’s Atomic Model