The Photon Nature of Light
The mean life of an excited state ranges from 10-7 to 10-10 sec, the excited electron returning to its previous state after the lapse of this time. In this transition, the atom must lose an amount of energy equal to the difference in energy between the two states that it has successively occupied, this energy appearing in the form of radiation. According to the postulates of Bohr, this energy is emitted in the form of a photon of light, the frequency if this radiation being given by E=hf. The term photon denotes an amount of radiant energy equal to the constant h times the frequency. This quantized nature of an electromagnetic wave was first introduced by Plank, in 1903, in order to verify theoretically the blackbody radiation formula obtained experimentally.
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| Energy Level In an Atom |
The photon concept of radiation may be difficult to comprehend at first. Classically, it was believed that the atoms were systems that emitted radiation continuously in all directions. According to the foregoing theory, however, this is not true, the emission of light by an atom being a discontinuous process. That is, the atoms radiates only when it makes a transition from one energy level to a lower energy state. In this transition, it emits definite amount energy of one particular frequency, namely, one photon hf of light. Of course, when a luminous discharge is observed, this discontinuous nature of radiation is not suspected because of the immense number of photons that are emitted in unit time.
As most loosely bound electrons of an atom are given more and more energy, it moves into stationery states which are farther and farther away from the nucleus. When its energy is large enough is large to move it completely out of the field of influence of ion, it becomes ‘detached’ from it. The energy required for this process to occur is called the ionization potential and is the highest state in the energy level. This is seen than mercury has potential of 10.39 eV. The alkali metal has the lowest ionization potential, whereas the inert gases have the highest values, the ionizing potential ranging from approximately 4 to 25 eV.
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