Content area
Full text
Every student of chemistry is taught at an early stage the achievements of Moseley (see eq 1 below). (equation omitted) Generally somewhat later students are taught about the spectrum of the hydrogen atom, the Bohr theory, and also Slater's rules for screening constants (eqs 2-7 below). The connection between these is often not clearly explained in textbooks. In this article I make this connection manifest. I also show, in the light of more modern data that ManualAds equation is actually an approximation, and I consider the significance of this fact.
In 1913, Moseley (1, 2) published (ref 3; compare with ref 6) his results on the K oc (2p ls) X-ray emissions for a number of elements; we present his data in column 1 of our table. They fitted the equation
(equation omitted)
where Z was the now-familiar atomic number; and R was the Rydberg constant.
From this he drew the following famous conclusion (3a).
We have here a proof that there is in the atom a fundamental quantity
Z, the atomic number
, which increases by regular steps as one passes from one element to the next. This quantity can only be the charge on the central positive nucleus, of the existence of which we already have definite proof.
Moseley quotes these final values to a precision of four significant figures. The values of Avogadro's number and the charge on the electron then available (these were used in the calibration) were only quoted to three significant figures. He observed diffraction lines in both second and third order (using a crystal of potassium ferrocyanide). Because the third-order lines were stronger and more accurate, he preferred to use those in constructing his table. Although the Kcl wavelengths for iron and copper from the second- and third-order lines differed by only 1 in the fourth significant figure, other discrepancies were larger; for calcium (experimentally particularly difficult, he commented) and titanium, there is a discrepancy of 1 even in the third significant figure. So the data strictly obeyed eq 1 with the experimental precision then available.
Historical and Theoretical Background
The theoretical background at that time led Moseley in the direction of the algebraic form of eq 1. Rydberg had recast Balmer's empirical formula for the lines in...