Thread 1 - Atomic Weights, then Atomic Numbers

Moseley and Atomic Numbers

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Leading up to Moseley: X-Ray Spectra

Henry Gwyn-Jeffreys Moseley was born on November 23, 1887 and would die in battle on August 10, 1915, before he turned 28. However, as long as our civilization stands, he will be remembered as the man who numbered the elements. That work, completed in a six-month span during 1913 and 1914 and publishd in the last two papers of his life was a tour de force of scientific accomplishment. Said Robert Milikan:

"In a research which is destined to rank as one of the dozen most brilliant in conception, skillful in execution, and illuminating in results in the history of science, a young man twenty-six years old threw open the windows through which we can glimpse the sub-atomic world with a definiteness and certainity never dreamed of before. Had the European War had no other result than the snuffing out of this young life, that alone would make it one of the most hideous and most irreparable crimes in history."

A brief summary of X-ray research is in order, since Moseley will use a regular change in the position of lines in the X-ray spectrum of each element to assign a positive charge (the atomic number) to the nucleus of each element.

I. The Discovery of Secondary X-Rays

Our X-ray thread starts in the evening of November 8, 1895. This is the day that Wilhelm Conrad Röntgen discovered X-rays. He realized the importance of his discovery at once. He stayed up all night doing experiments and even ate and slept in the laboratory for a time. His "preliminary communication" on X-rays was turned in on December 28, 1895 and published before the end of the year. Very quickly, others began studying X-rays, with many new discoveries being made.

For us, the next step in our story was made in 1897. It was found that, when a primary X-ray beam was directed at a substance, that substance gave off secondary X-rays. (Please realize that many other discoveries were made about X-rays. I'm just highlighting the ones which culminate in Moseley's work.)

II. Secondary X-Rays are Characteristic of the Element

The next discovery was made by Charles G. Barkla. He found a connection between the atomic weight of the element and its secondary X-rays. His first efforts in this area were in 1906 (the same year Rutherford discovered alpha-particle scattering) and in 1909 he wrote:

"It has been found that each of the elements Cr, Fe, Co, Ni, Cu, Zn, As, Se, Ag, when subject to a suitable primary beam of X-rays, emits an almost perfectly homogeneous beam of X-rays, the penetrating power of which is characteristic of the element emitting it."

Barkla ordered the list of elements above by the penetrating power of the secondary radiation with the Cr called "soft," which means not very penetrating, up to silver which is very penetrating or "hard." Notice that the list follows the chemical order of Co then Ni. If the list were ordered by strict atomic weights, the Ni would come first.

Since no one could yet measure the wavelengths (or frequencies) of X-rays, Barka measured the absorbance of each secondary radiation. He did so by directing the secondary radiation through a 0.01 cm layer of aluminum and measuring how much of the beam was absorbed.

It turns out that the "hard" radiation (the more penetrating ones) has the shortest wavelength (which also means the highest frequency and the highest energy). So as the atomic weight increased (with the Co/Ni exception), the secondary X-ray became harder and harder. "Soft" X-rays means of lower penetrating ability, so much of the secondary beam is absorbed. (Soft means longer wavelength X-rays which also means lower frequency and lower energy.) In the early years, elements below about aluminum could not be studied due to the instruments not being sensitive enough to measure the X-rays after absorption.

III. A Second X-Radiation is Found

Barkla (and his students) continued the detailed study of secondary X-radiation. In 1909, Barkla published another paper in which he found that the supposedly homogeneous secondary X-rays were, in fact, heterogeneous. He wrote:

"The writer has recently investigated more closely the radiations from Sn, Sb, I (which have been recorded as elements emitting a radiation of variable penetrating power). It has been found that these consist of a very easy absorbed radiation and a very penetrating homogeneous radiation superposed. The absorptions of the penetrating portions of the beams from each element are shown in fig. I on curve B. The percentage absorptions of the soft radiations from these elements have not yet been determined, but they are roughly indicated on curve A in fig. 1. Though a full analysis of the radiations from W, Pt, Au, Pb, Bi, etc., has not yet been made, there is strong evidence that the observed radiations from these elements are also principally homogeneous radiations characteristic of the elements emitting them."

In 1911, Barkla wrote:

"It is seen that the radiations fall into two distinct series, here denoted by the letters K and L*."

In the footnote indicated by the asterisk, he added:

"* Previously denoted by letters B and A. The letters K and L are, however, preferable as it is highly probable that series of radiations both more absorbable and more penetrating exist.

Barkla closed his paper this way:

"It has been shown that each element has its own characteristic fluorescent line spectrum in X-rays. This is very conveniently represented as is [in?] a spectrum of ordinary light, except that without a knowledge of the wave-length we are obliged to define the radiations by their absorption in some standard substance. Thus we may represent the known portion of the spectra of elements Sb, I, and Ba as in fig. 5. The lines move towards the more penetrating end of the spectrum with an increase in the atomic weight of the element.

It is scarcely too much to say that all the phenomena connected with the transmission of X-rays through matter may be readily explained in terms of a few simple laws expressed with reference to these spectra."

Barkla wrote this slightly two years before Moseley would publish his historic papers in December 1913 and April 1914.

Thread 1 - Atomic Weights, then Atomic Numbers

Moseley and Atomic Numbers

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