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Monday, 6 February 2017

Structure of Atom III


Black body: An ideal body, which emits and absorbs all frequencies, is called a black body. The radiation emitted by such a body is called black body radiation.

Photoelectric effect: 

The phenomenon of ejection of electrons from the surface of metal when light of suitable frequency strikes it is called photoelectric effect. The ejected electrons are called photoelectrons.

Experimental results observed for the experiment of Photoelectric effect :

  • When beam of light falls on a metal surface electrons are ejected immediately. 
  • Number of electrons ejected is proportional to intensity or brightness of light.
  • Threshold frequency (vo): For each metal there is a characteristic minimum frequency below which photoelectric effect is not observed. This is called threshold frequency.
  • If frequency of light is less than the threshold frequency there is no ejection of electrons no matter how long it falls on surface or how high is its intensity.
Photoelectric work function (Wo): The minimum energy required to eject electrons is called photoelectric work function.

Energy of the ejected electrons :

Wo = hvo  
Dual behavior of electromagnetic radiation:
  • The light possesses both particle and wave like properties, i.e., light has dual behavior. 
  • whenever radiation interacts with matter, it displays particle like properties.
  • (Black body radiation and photoelectric effect) Wave like properties are exhibited when it propagates(interference an diffraction)
  • When a white light is passed through a prism, it splits into a series of coloured bands known as spectrum.
Spectrum is of two types: continuous and line spectrum :
a. The spectrum which consists of all the wavelengths is called continuous spectrum.
b. A spectrum in which only specific wavelengths are present is known as a line spectrum. It has bright lines with dark spaces between them.

Electromagnetic spectrum is a continuous spectrum. It consists of a range o of electromagnetic radiations arranged in the order of increasing wavelengths or decreasing frequencies. It extends from radio waves to gamma rays.


Spectrum is also classified as emission and line spectrum :

Emission spectrum: 
  • The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum.
  • Absorption spectrum is the spectrum obtained when radiation is passed through a sample of material. The sample absorbs radiation of certain wavelengths. The wavelengths which are absorbed are missing and come as dark lines.
  • The study of emission or absorption spectra is referred as spectroscopy. 
Spectral Lines for atomic hydrogen:

Rydberg equation


R = Rydberg’s constant = 109677 cm-1

Bohr’s model for hydrogen atom:

a. An electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits or energy levels. These orbits are arranged concentrically around the nucleus.
b. As long as an electron remains in a particular orbit, it does not lose or gain energy and its energy  remains constant.
c. When transition occurs between two stationary states that differ in energy, the frequency of the radiation absorbed or emitted can be calculated.
 
d. An electron can move only in those orbits for which its angular momentum is an integral multiple of h/2π
  • The radius of the nth orbit is given by rn =52.9 pm x n2/Z
  • energy of electron in nth orbit is:


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