In 1913, Neil Bohr proposed an atomic model based on the Planck’s quantum theory. The basic postulates of this theory are given below.
- An atom consists of positively charged nucleus in the centre surrounded by negatively charged electrons. The electron in an atom revolve around the nucleus only in a certain selected path called orbit.
- The energy of an electron in an orbit always remains constant. So they are also called as energy level or shell. These are numbered as 1,2,3….. and designated as K, L, M, N etc.
- As long as electron revolves in an orbit, it neither gains nor loses energy. This means that the energy of the electron remains constant in an orbit.
- Only those orbits are permitted in which the angular momentum of the electron is whole number multiple of the quantity as by required given by: mvr = nh/2π where,
m = mass of electron
v = velocity of electron
r = radius of the orbit
n = whole number integer
h = planks constant
- When energy is supplied to an atom, the electron absorbs energy and jumps to higher energy level. As soon as the supplied energy is cut off, the electron jumps back to its original energy level. This transition of electron form one energy level to the another energy level is called accompanied by either absorption or emission of radiation.
Success of Bohr’s atomic model:
- It explains the stability of the atom.
- It helps to calculate the atomic radius of by the formula.
rn = 0.53 × n2, where n is the number of orbit located.
- It can be used to determine the energy of the electron in an atom. (En = -13.6/n2 ).
- It can be used to explain the hydrogen spectra.
Drawbacks of Bohr’s atomic model:
- It cannot explain the line spectra of multi electron system.
- It cannot explain finer spectra of atoms.
- It cannot explain the splitting of lines in presence of electric field and magnetic field.
- Bohr’s idea about planetary orbit is not true as electrons are present all over the nucleus in three dimensional space.
- It does not take in account the D-Broglie wave particle nature of electron and Heisenberg’s uncertainty principle.