Moseley Experiment and Moseley’s Law : Statement and Mathematical Expression

Moseley Experiment :

Henry Moseley in 1913 performed an experiment and showed that the frequencies of emitted x-rays increase regularly with the number of positive charges in the nucleus of target elements, used as anode. His experiment became bases to determine atomic numbers. 

Experimental set up of Moseley’s experiment is shown in figure below.      

X-Rays and Atomic number (Z) Relation

He performed a number of experiments at the University of Manchester, in 1913. He used different anodes and took spectrum of x-rays in each case, by allowing them to fall on a photographic plate.

Relationship between X-Rays and Atomic number (Z):

            He observed that the wavelengths of x-rays were the characteristics of each element, used as anode. The characteristic x-ray spectrum consists of discrete spectral lines which can be grouped into K-series, L-series, M-series, N-series etc.

The number of lines in x-ray spectrum depends upon:

  • the nature of the target material i.e. the nuclear charge of target material.
    • the excitation voltage.

Moseley’s Law :

Statement & Expression :

Where “a” is proportionality constant and its values depends on the metal under consideration. This is a straight line equation and is useful for calculations of atomic number Z. By plotting graph of square root of frequencies of x-rays verses atomic numbers of elements we get straight line and from it atomic numbers of unknowns can be determined. 

Further more it is concluded that frequency and wavelength of x-rays is function of atomic number of the metal. This is the basis of modern periodic law.

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