Moseley՚s Law, Introduction to Moseley՚s Law, Experiment, Analysis (For CBSE, ICSE, IAS, NET, NRA 2022)

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Moseley՚s Law

  • Henry Moseley Discovered That the Wavelength (Energy) of an X-Ray Depends on the Nuclear Charge of an Atom.
  • He Photographed the X-Ray Spectrum of Ten Elements That Occupied Successive Places in the Periodic Table, in 1913.
  • He Accomplished That There Was a Fundamental Quantity Which Increases by Regular Steps as We Pass from One Element to the Next.
  • Rutherford Recognized this Quantity as the Atomic Number in 1920

Introduction to Moseley՚s Law

  • Soon After Rutherford՚s Scattering Theory Had Been Confirmed by Experiment, the One-to-One Association of an Atomic Number Z with Each Element Was Solidified by the Work of Henry Moseley
  • He Used Bohr՚s Model of Atomic Structure to Determine the Energy Emitted when Low-Level Electrons Change Orbital.
  • This Energy Has a Strong Dependence on an Atomic Number So That by Measuring the Energy of the X-Rays Characteristic of an Element, Its Atomic Number Z Can Be Unambiguously Determined.
  • This Law is an Empirical Law Concerning the Characteristic X-Rays That Are Emitted by Atoms.
  • The Kα Lines Shifted to Higher Energy with Increased Atomic Number Because of the Inner Shell Electrons Are More Tightly Bound by the Higher Number of Protons in the Nuclei.
  • Accurate Mathematical Equation between Frequency and Atomic Number is Given by

  • Where Z is an Atomic Number of the Element and a and B Are Const
  • A widespread simplification is an idea that the effective charge of the nucleus decreases by 1 when it is being screened by an unpaired electron that remains behind in the K-shell.
  • Statement: The frequency of a spectral line in the characteristics X-ray spectrum varies directly as the square of the atomic number of the element emitting it.
  • Moseley՚s diagram for lines, which is obtained by plotting versus atomic number of different elements of the periodic table.
  • The graph is linear as expected.

Moseley՚s Diagram for Lines

Moseley՚s Diagram for K_α and K_β Lines


  • The detector will see more than just the x-rays of interest to you. In addition, -rays directly from the Co source will strike the detector.
  • More importantly, source will also interact in the shielding, producing a lead x-ray spectrum.
  • Though There is so much more the lead shielding than there is a sample, this may mask the signal.
  • For eliminating this background, the data are accumulated in two steps. First, with the multi-channel analyzer on add, data are accumulated for some predetermined time. Then the sample is removed and the analyzer is set to subtract.
  • Data are accumulated for the same amount of time. This eliminates the background counts at the same rate they were accumulated, resulting in data which represent only the contribution from the sample.
  • Measure the x-ray spectra of six known samples Firstly: Al, Ti, Cu, Zr, Ag, and Te. Save your data to files that you can read later.
  • Measure the spectra from a two of unknown elements. Again, save your data to files that you can read later.


You should confirm Moseley՚s law with six known samples. Since the energy of the characteristic x-ray should (according to Moseley) be proportional to and channel number N is proportional to E, then N is proportional to .

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