Lesson

        Chemistry deals with structure, composition, properties of matter. Matter can be classified into mixtures, compounds and elements. All of them consists molecules and atoms. According to Greek philosophers atoms are fundamental blocks of the matter. The word atom was derived from Greek word "atomio", which means "non-divisible". According to Daltons atomic theory, atoms can be further divided into sub atomic particles like protons, electrons and neutrons. This theory successfully explained the law of Conservation of mass, Law of Constant composition and law of multiple proportion.

       Electron was discovered by J.J. Thomson in cathode rays experiment in a discharge tube. Proton was discovered by Goldstein in canal ray's experiment. Neutron was discovered by James Chadwick by bombarding a thin sheet of Be by α -particles. Experimental observations suggested few atomic models to explain the structure of atom i.e., distribution of these charged particles in an atom.
 

 According to Thomson watermelon model of atom, positive charge is uniformly distributed in an atom just like red mass present in watermelon and the electrons are embedded in it (like seeds). Rutherford's nuclear model of atom resembles the solar system. In which nucleus is compared with Sun and electrons with the revolving planets.  This model could not explain the stability of atom and electronic structure of atom. 

 Maxwell suggested that when electrically charged particle moves under acceleration, electro-magnetic waves are produced. They are propagating in perpendicular directions in one other, can move in vacuum. The spectrum of electromagnetic radiation consists γ-rays, X-rays, u.v. rays visible I.R., microwave, radio and long radio waves.
 

 The distance between two successive crests or troughs in a wave is called wavelength (λ). The number of waves passing through a point in one second is called frequency (ν). The number of waves present in Unit length are called wave number (). The height of the crest or depth of a trough is called amplitude.
             

   Velocity of light (c) = ν . λ          

 Light is electromagnetic radiation which possesses both the particle nature (black body radiation & photo electric effect) and wave nature (diffraction and interference). Particle nature of electromagnetic radiation was explained by Plank's quantum theory.

Plank's quantum theory:
    Black body radiation was successfully explained by Max Plank in 1900. A hallow metallic sphere coated inside with platinum black with a fine hole acts as black body. Which is a perfect absorber and perfect radiator of energy. Radiation is associated with energy. The energy is emitted or absorbed by a body discontinuously in the form of a small packet called "quantum". Energy is directly proportional to the frequency of radiation.   E   ν
                   Therefore    E = h ν
       Propagation of radiant energy in the form of quanta is called "quantization of energy".
          E = n . h ν (n = integer)
      Curves are obtained by plotting 'E' against 'λ'. As the temperature increases, the peak of the curve shifts to lower λ.
At a given temperature, the intensity of the radiant energy increases with the λ, reaches maximum and then decreases.
        In 1905, Einstein replaced the word quantum by "photon". He explained photo electric effect. Emission of electrons from a clean metal surface when light (photons) with suitable λ falls on it is called "photo electric effect". It is readily exhibited by alkali metals.
                  h ν = W + K.E.
                  h ν = energy of photon
                  W = work function
                  K.E. = Kinetic energy of emitted electron.
     The series of coloured bands obtained by splitting of electromagnetic radiation when it is passed through prism is called "spectrum". The spectrum which is produced due to excitation and de-excitation of electrons of atoms is called "line spectrum". This spectrum has sharp, well defined and distinct lines. If the spectrum is produced by molecules, it is called "Band spectrum", which has closely spaced lines (bands). The study of emission or absorption spectra is known as "spectroscopy" and is useful in chemical analysis.

"Absorption spectrum" is produced due to excitation of atoms or molecules or ions, when they absorb energy. This spectrum consists dark lines on a bright background. "Emission spectrum" is produced due to emission of light by excited ions or atoms or molecules. This spectrum consists of bright lines on a dark background.

 Bohr's Model of Atom

        Bohr's model of atom is a modification of Rutherford's model. It is based on Plank's quantum theory and hydrogen spectrum.
 

Postulates:      

¤ Electrons revolve round the nucleus in fixed, circular paths called 'orbits'.
  ¤ Orbits are denoted by 1, 2, 3, 4... or K, L, M, N ...
  ¤ Each orbit is associated with definite amount of energy called Energy levels.
 ¤ Electrons neither emit nor absorb energy when they revolve in a orbit called 'stationary orbit'.
 ¤ As 'n' value increases the size, energy of orbit increases. The angular momentum of an electron is integral multiple of   i.e. mvr = 
 ¤ Energy absorbed or emitted by electron is given by ∆E = E2 - E1 = hν.

Explanation of emission spectrum of H-atom:

       Of all the atomic spectra, the hydrogen spectrum is the simplest spectrum. H atom has only one electron, but it gives 5 series of spectral lines. When H2 gas is heated or exposed to light or subjected to electric discharge, electrons of different H atoms get excited to different higher orbits and deexcites to different lower orbits in different manner with the emission of energy and give 5 series of spectral lines. λ of a spectral line in H atom can be calculated by using Rydberg's equation