Moseley's law

Henry Moseley, holding an X-ray tube

The historic periodic table was roughly ordered by increasing atomic weight, but in a few famous cases the physical properties of two elements suggested that the heavier ought to precede the lighter. An example is cobalt having the atomic weight of 58.9 and nickel having the atomic weight of 58.7.

Henry Moseley and other physicists used X-ray diffraction to study the elements, and the results of their experiments led to organizing the periodic table by proton count.

Since the spectral emissions for the lighter elements would be in the soft X-ray range (absorbed by air), the spectrometry apparatus had to be enclosed inside a vacuum.[4] Details of the experimental setup are documented in the journal articles "The High-Frequency Spectra of the Elements" Part I[1] and Part II.[2]

Moseley found that the ${\displaystyle K\alpha }$ lines (in Siegbahn notation) were indeed related to the atomic number, Z.[2]

Following Bohr's lead, Moseley found that for the spectral lines, this relationship could be approximated by a simple formula, later called Moseley's Law.[2]

$${\displaystyle \nu =A\cdot \left(Z-b\right)^{2}}$$

where:

• ${\displaystyle \nu }$ is the frequency of the observed X-ray emission line
• ${\displaystyle A}$ and ${\displaystyle b}$ are constants that depend on the type of line (that is, K, L, etc. in X-ray notation)
• ${\displaystyle A=\left({\frac {1}{1^{2}}}-{\frac {1}{2^{2}}}\right)\cdot }$ Rydberg frequency and ${\displaystyle b\ }$= 1[2] for ${\displaystyle K\alpha }$ lines, and ${\displaystyle A=\left({\frac {1}{2^{2}}}-{\frac {1}{3^{2}}}\right)\cdot }$ Rydberg frequency and ${\displaystyle b=7.4}$[2] for ${\displaystyle L\alpha }$ lines.

Moseley derived his formula empirically by fitting the square root of the X-ray frequency plotted against the atomic number.[2] This formula can be explained based on the Bohr model of the atom, namely,

$${\displaystyle E=h\nu =E_{\text{i}}-E_{\text{f}}={\frac {m_{\text{e}}q_{\text{e}}^{2}q_{Z}^{2}}{8h^{2}\varepsilon _{0}^{2}}}\left({\frac {1}{n_{\text{f}}^{2}}}-{\frac {1}{n_{\text{i}}^{2}}}\right),}$$

where

• ${\displaystyle \varepsilon _{0}}$ is the permittivity of free space
• ${\displaystyle m_{\text{e}}}$ is the mass of an electron
• ${\displaystyle q_{\text{e}}}$ is the charge of an electron
• ${\displaystyle q_{Z}}$ is an effective charge of the nucleus, expressed as ${\displaystyle (Z-b)q_{e}}$
• ${\displaystyle n_{\text{f}}}$ is the quantum number of final energy level
• ${\displaystyle n_{\text{i}}}$ is the quantum number of initial energy level (${\displaystyle n_{\text{i}}>n_{\text{f}}}$)

Taking into account the empirically found b constant that reduced (or "screened") the nucleus charge, Bohr's formula for ${\displaystyle K\alpha }$ transitions becomes[2]

$${\displaystyle E=h\nu =E_{\text{i}}-E_{\text{f}}={\frac {m_{\text{e}}q_{\text{e}}^{4}}{8h^{2}\varepsilon _{0}^{2}}}\left({\frac {1}{1^{2}}}-{\frac {1}{2^{2}}}\right)(Z-1)^{2}\approx {\frac {3}{4}}(Z-1)^{2}\times 13.6\ \mathrm {eV} .}$$

Dividing both sides by h to convert to the frequency units, one obtains

$${\displaystyle \nu ={\frac {E}{h}}={\frac {m_{\text{e}}q_{\text{e}}^{4}}{8h^{3}\varepsilon _{0}^{2}}}{\frac {3}{4}}(Z-1)^{2}\approx (Z-1)^{2}\times (2.47\cdot 10^{15}\ \mathrm {Hz} ).}$$

A simplified explanation for the effective charge of a nucleus being one less than its actual charge is that an unpaired electron in the K-shell screens it.[5][6] An elaborate discussion criticizing Moseley's interpretation of screening can be found in a paper by Whitaker[7] which is repeated in most modern texts.

A list of experimentally found and theoretically calculated X-ray transition energies is available at NIST.[8] Nowadays, theoretical energies are computed with a much greater accuracy than what Moseley's law provides, using modern computational models such as the Dirac–Fock method (the Hartree–Fock method with the relativistic effects accounted for).

• Moseley's periodic law, concerning the modern periodic table
• Auger electron spectroscopy, a similar phenomenon with increased X-ray yield from species of higher atomic number

• Oxford Physics Teaching - History Archive, "Exhibit 12 - Moseley's graph Archived 2016-03-03 at the Wayback Machine" (Reproduction of the original Moseley diagram showing the square root frequency dependence)