Structure of Atom
11.0 Atomic Spectra
11.0 Atomic Spectra
- Ordinary white light consists of waves with all the wavelength of visible range.
- When a ray of white light is passed through a prism, it dispersed to form a series of coloured bands called as spectrum.
- Red colour light with longest wavelength deviates least and violet colour light with shortest wavelength deviates most.
- The spectrum of white light is a continous spectrum because it consists of all ranges of frequency from violet at $7.5 \times {10^{14}}$ Hz to red at $4 \times {10^{14}}$ Hz.
Emission and Absorbtion Spectra
- Emission spectrum is obtained when any substance which has absorbed energy emitted the radiation.
- An absorption Spectrum is observed when white light is passed through the sample substance which absorbs radiation of certain wavelengths. These missing wavelengths creates dark lines in the bright continous spectrum.
- The emission spectra of atoms in the gas phase, do not show a continuous spread of wavelength from red to violet, rather they emit light only at specific wavelengths with dark spaces between them. Such spectra are called line spectra or atomic spectra because the emitted radiation is identified by the appearance of bright lines in the spectra.
Line Spectrum of Hydrogen
- The hydrogen spectrum consists of several series of lines.
$$\overline \nu = \frac{1}{\lambda } = R\left( {\frac{1}{{n_i^2}} - \frac{1}{{n_f^2}}} \right)$$
- The value R=109677 /cm is called as Rydberg constant.
- ${n_i}$=1,2,3,4,5 are known as Lyman, Balmer, Paschen, Bracket, Pfund respectively.
- For calculation purpose, we use $\frac{1}{R} = 912\mathop {\text{A}}\limits^ \circ $
