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3055- Chapter 8: x-rAY sPECTROMETRY
Moseley’s
law for a series of homologous lines (1913)
Moseley examined metals from aluminium to gold and he
found that their X-ray spectra were similar but with a deviation that changed
regularly through the series, according to Equations below. Here, Z is the atomic number (i.e. number of
positive charges in the nucleus); and k
are constants depending on the type of X-ray line (K or L or M etc.-Fig. 1).
Formulation in terms of frequency (
): the square root of the frequencies of lines in
a series of x-ray spectra depends linearly on the atomic number of the emitting
atom.
Formulation in terms of wavelength
(l).
As 
(c = light speed) it results:
Fig.
1. Plot of
Moseley's law, showing dependence of characteristic X-ray-line wavelengths on
atomic number Z.
Moseley's
law is an empirical law concerning the characteristic x-rays that are emitted
by atoms. It is historically important in quantitatively justifying the
conception of the nuclear model of the atom, with all positive charges of the
atom located in the nucleus, and associated on an integer basis with atomic
number. Until Moseley's work, "atomic number" was merely an element's
place in the periodic table, and was not known to be associated with any
measureable physical quantity. See Fig. 2 for the original
plot (H. G. J. Moseley, M. A., Phil. Mag.
(1913), p. 1024)
The Bohr model
of the atom provides a theoretical
background to Moseley's
law
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Fig. 2. Moseley’s law, as plotted by the discoverer himself. Source
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