Planks Quantum Theory Rutherfords atomic model and Bohrs atomic theory

Atomic structure

• The arrangement of fundamental particles in the atom is called atomic structure.
• Various atomic models were proposed by different scientists.
• Rutherford proposed atomic model based on the α- ray scattering experiment.

QUANTUM THEORY OF RADIATION

• In 1901 Max Plank proposed Quantum theory of Radiation from hot (black) body studies.
• A hot body radiates (emits) energy not in continuous waves but in small units of waves.
• When atoms or molecules absorb or emit radiant energy in quanta (a small packet of energy) or photons.

    RUTHERFORD’S MODEL OF ATOM

• Atom is spherical and mostly hollow.
• The positive charge of the atom is concentrated in a small region called as nucleus.
• Protons and neutrons are present in the nucleus.
• Protons and neutrons are called Nucleons
• The electrons revolve round the nucleus like planets do around the Sun.
• Nuclear model fails to explain 

          (a) Stability of atom, and

          (b) Line spectrum of atoms

   BOHR’S ATOMIC THEORY

• Neils Bohr postulated a theory based on the Planck’s quantum theory.
• According to Planck’s quantum theory “energy is radiated not continuously but discontinuously in energy packets called quanta given by the relation.
• The energy of Photon: The energy, E, of a photon is given by the Einstein relation

    Postulates of Bohr’s atomic theory :

1.   Electrons in an atom revolve round the nucleus in circular orbits
2.   Each orbit is associated with definite energy. So these orbits are also called energy levels.
3.  Orbits are numbered as 1,2,3,4 etc or K,L,M,N etc starting from the nucleus
4.  The energy of emitted or absorbed photon is equal to the difference in energies of the two orbits

                ΔE=E2-E1 = hν

                where ΔE   = difference in energy

                            E1  = energy of lower orbit

                            E2  = energy of higher orbit

                               h = planck’s constant

                               ν = frequency of radiation

5. The angular momentum of revolving electron is integral multiple of h / p2

           por mvr = h/2pmvr = nh/2

           (m = 1,2,3,4….)

           where m = mass of electron

           v = velocity of electron

           r = radius of the orbit

           h = planck’s constant

           n = principal quantum number

6.Centrifugal force of the electron is balanced by the electrostatic attraction between nucleus and electron. 

 LIMITATIONS OF BOHR’S ATOMIC THEORY:

1. Bohr’s Atomic Theory fails to explain the spectral details of multi electron atoms.
2. Bohr proposed a flat model for atom, but it is that it has 3-dimensional existence.
3. It fails to explain the splitting of a spectral line under the influence of a magnetic field (Zeeman effect) or electric field (stark effect).