Spectroscopy, Electromagnetic Radiations and Types of Spectra | Chemistry Notes Info - Your Chemistry Tutor provide notes for Classes, BSc, MSc, Chemistry Test

Spectroscopy, Electromagnetic Radiations and Types of Spectra

SPECTROSCOPY

Spectroscopy

It is a branch of science which deals with the study of interaction of matter with electromagnetic radiation.

Electromagnetic Radiation

Electromagnetic Radiations are a certain amount of energy depending upon its wavelength. Hence these radiations are the sources of energy so called as electromagnetic energy or radiant energy.
Electromagnetic Radiations are of many types like radio waves, UV rays, IR rays, visible light etc. these are all electromagnetic radiations with different energy, wavelength and frequency. All types of electromagnetic radiations travel with the speed of light but differ in wavelength and frequency.
Energy of electromagnetic radiations can be expressed as
                       
Where E = Energy
            n = Frequency
            l = Wavelength
            c = Velocity of light = 3*108 m/s.
            h = planks constant
            Types of electromagnetic radiations
Type
Frequency (Hz)
Wavelength (1/m)
Radio wave (low energy)
3*105
103
Microwave
3*109
10-1
Infrared
3*1010
10-2
Visible light
3*1014
10-6
Ultraviolet
3*1016
10-8
X-rays
3*1018
10-10
Gamma rays (high energy)
>3*1018
<10-10

Type of spectra :  

Spectra are of two types

1.     Emission spectra : 

 When a substance is exposed to intense heat or light its atoms or molecules absorbs energy and get excited. When these atoms or molecules return to their initial state they emit radiations which when pass through a prism it produce a spectrum. This spectrum is known as a emission spectrum. When this type of spectrum is recorded on a photographic plate then bright lines are formed on a black background in case of atoms and bands are formed in case molecules

2.     Absorption Spectra :  

     When a substance or matter is exposed to intense heat or light it absorbs energy. Due to this intensity of absorption varies as function of the frequency. This variation is known as absorption spectrum. This type of spectra contains dark (black) lines or bands on light coloured background as some radiations are absorbed by the medium.
In this process only those photons of radiation are absorbed whose energy is equal to energy difference (DE) between two energy levels of molecules of substance.
                                   
Where h = planck’s constant = 6.63 * 10-34 Js
            n = Frequency
            c = Velocity of light = 3 * 108 m/s
            l = Wavelength of radiation
            NA = Avogadro’s number = 6.02 * 1023 mol-1

Difference between Emission and Absorption Spectrum : 

In Emission Spectrum molecules come back to lower energy state from higher energy state while in Absorption Spectrum The molecule goes to Higher energy state from lower energy state.

Absorption Spectrum and its types :  

in addition to the nuclear energy, total internal energy of a molecule consist of three types of energies.
                                    Einternal = Eelectronic + Evibrational + Erotational
Since, electromagnetic radiation is a form of energy so its absorption by a molecule, increases the internal energy of the molecule. Also when a molecule is exposed to electromagnetic radiation, molecule does not absorbs all the radiations rather it absorb a particular portion of radiation depending upon the structure of the molecule and amount of absorption of energy depend upon the frequency of the radiation also. So it is clear that different molecules absorb different type of energies and undergo different excitations. Depending upon the absorption of energy molecule may go under Electronic excitation, Vibrational excitation and/or Rotational excitation. So produces different Spectra.
 
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Table : Some Components of the Electromagnetic Radiation and Absorption Spectra
Absorbed Radiation and Type of Spectra
Wavelength
Energy
(kJ/mol)
Types of Excitation
Effect on Molecule
Application
Microwave
1 cm
0.01 - 1
Rotational
Changes in the Rotational energy levels of the molecule
Calculation of bond distance and bond length
Infrared
2 – 15 mm
1 – 100
Vibrational and Rotational
Changes in Vibrational and rotational energy levels
Identification of functional groups, Calculation of Bond length, Bond angle and Qualitative analysis
Electronic
(i)                Visible
(ii)              Ultraviolet
200-400 nm
400-800 nm
150-300
300-600
Electronic
Vibrational and rotational transitions also take place but their resolution is not measurable
Change in Electronic energy level
Qualitative and quantitative analysis
The Absorption energy is measured with the help of Spectrophotometer and expressed as Frequency, Wavelength or Wave-number.
                       

Law of Light Absorbance :  

According to the Lambert Beer’s Law “The Absorption of light is directly proportional to the concentration of the solution and the length of the cell containing sample.
                                   
                                      
Where, I0 = Intensity of incident Radiation
            I  = Intensity of transmitted radiation
            c = Molar concentration
            l = Length of the cell in centimetre
            e(epcylon) = Molecular extinction coefficient
 The Wavelength at which molecule has maximum Absorption coefficient e (max) is expressed as l (max). in above expression log I0/I is known as absorbance or optical density of the solution. And log I/I­0 is known as Transmission of the solution.
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