Remembering Henry Moseley (1887-1915)

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The 11th of November 2018 at 11 am is being remembered solemnly all the over the world as the centenary of the armistice of World War I. In the memory of recent generations, this conflict was the most dreadful that could be imagined. The casus belli was the most senseless. Nevertheless, the sacrifices made by so many, in the name of the political and personal freedoms, that we currently enjoy, were the noblest.

In viewing some of the coverage on TV and in the newspaper, many commentators were remembering notable individuals who lost their lives in that awful conflict. The person that I’d most like to remember this day is the British physicist Henry G. J. Moseley who left his work at Manchester University to volunteer for the Royal Engineers of the British Army.

His decision to volunteer was made in typical humility, as he likely would have heard from a number of his colleagues that they thought he was a candidate for the Nobel Prize in 1916. At age 27 years, such as was the stature of his contributions to our understanding of atomic physics, as we’ll get to shortly.

Moseley had the spirit and courage of a true pioneer in science, coupled with great original ability and powers of work. It is rare in the history of science that so young a man had achieved so much  – Sir Ernest Rutherford

He found himself assigned as a signals officer to the invasion force of British Empire soldiers, including many ANZAC soldiers from Australia and New Zealand Corps, in the Gallipoli Campaign, modern-day Turkey in April 1915. On August 10, 1915, the man known to his friends as “Harry” he was hit by a sniper’s bullet and died. One of 41,148 British soldiers killed in the campaign [1] that was ultimately aborted 9 Jan 1916. Unfortunately, there is no provision in the Nobel Prize for award posthumously.

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Henry Moseley ca. 1910

The photograph of Henry Moseley (shown) was first published in the journal Nature by Sir Ernest Rutherford on the occasion of the 10th anniversary of his passing [2]. It shows Moseley at the Balliol-Trinity Laboratory, Oxford in 1910. The apparatus he appears to be holding is a prototype cloud chamber that he was working on under the direction of C. T. R. Wilson at that time.

This point in Moseley’s short career was marked by hectic change in the understanding of the atom and matter. The concept that atoms were “indivisible” particles of matter, that had existed since ancient Greek civilisation, was unravelling.

Scientific Background

In 1897, experiments by J. J. Thompson had revealed that atoms were made up of negatively-charged particles (later understood to be electrons) that were very tiny when compared with the scale of the atom.

Between 1909 and 1914,  Ernest Rutherford directed a series of experiments by Hans Geiger and Ernest Marsden that fired positively charged alpha particles at gold foil to observe their scattering. Most of the alpha particles were hardly scattered at all. This indicated that the atom consisted of mostly empty space.

However, the most surprising observation was  that a small number of alpha particles were “backscattered.” That is, they were scattered back toward the source. This is like having bullets, fired from a gun, ricochet backwards, towards the gun itself.

The Rutherford atom
The Rutherford model of the atom.

The implication of these experiments was that the atom consisted of a cloud of negatively charged electrons surrounding a positively-charged, small dense nucleus (see image).

At the same time as Rutherford, Niels Bohr was also developing his model of the atom. In addition to the nucleus, the Bohr model consisted of discrete electron “shells” distinguished by “quantised” energy differences between the shells.

The nature of the nucleus would not be not fully understood for some years. The proton wasn’t characterised as the positively-charged nuclear particle until 1920. Likewise, it wasn’t until the discovery by James Chadwick in 1932, that the neutron became recognised as the other constituent of the nucleus, alongside the proton.

Moseley’s Contribution

Around 1913, as related by Ernest Rutherford in the article in Nature [2], Moseley proposed to settle the scientific question of whether the elements were ordered by atomic weight according to the periodic theory, put forward by Mendeleev, or the nuclear theory, based upon the new (to him) understanding of the atom outlined above. The nuclear theory stated that the ordering of the elements was based upon a whole number Z, equivalent to the number of positive charges within the nucleus of that element.

Moseley proposed to use the X-ray techniques and methods developed by father and son William and Lawrence Bragg [3], in the period 1912–1913, at the University. of Leeds. He irradiated metal samples with an X-ray tube, then measured the frequency of the X-rays that were re-emitted from the sample using a spectrometer of his own construction.

The technique chosen by Moseley uses two types of X-rays: X-rays generated by an X-ray tube, known as the “source” and secondary X-rays generated when the source X-rays strike a “target.” These secondary X-rays have different frequencies from the primary source, hence they can be distinguished using a spectrometer that splits the X-rays into different spatial planes according to their frequency.

Once thus split, the X-rays are recorded as “lines” on a photographic plate. Knowing the angular separation between these lines, with regard to the diffracting element of the spectrometer, the frequency of the radiation, that each line represents, can be determined very precisely, using the mathematical relationship established by Lawrence Bragg, known as Bragg’s Law.

Rutherford noted in the article Nature [2}, that William and Lawrence Bragg performed similar experiments, with metal targets, and had observed bright lines on their photographic plates. But the Braggs had not continued to investigate this phenomenon systematically in the way that Moseley was about to do in 1913.

Characteristic X-ray Spectra

A reproduction of photographic plates from Moseley’s experiments, showing secondary X-rays emission in two spectral lines, known as Kα and Kβ, are reproduced below, the photographic plates are organised by frequency (X-axis) and by target material (Y-axis).

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Photographic plates from Moseley’s experiments organised by frequency (X-axis) and by target material (Y-axis).

The important scientific observation was that the X-ray spectra were characteristic for each element. Knowing the X-ray emission frequencies was enough to unambiguously assign the element present or, as in the case of brass, both of the elements present together, as brass is made up of Cu and Zn. (As an aside, the precise nature of the spectral splitting of the K lines into Kα and Kβ components was not understood at Moseley’s time [4].)

When you compare the photographs above with a modern-day periodic table you can see (diagram below) that the order is reversed with regard to atomic number. Noting that Moseley used a brass target for Zn and that Sc is missing – samples of pure scandium were rare in 1913.

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The precise mathematical relationship can be obtained by plotting Z (on the Y-axis) against the square root of the frequency (on the X-axis) as in the graph below. Moseley's Law PlotAs shown by the graph above [5] the relationship between the X-ray spectra of the elements is in near-perfect agreement with the square-root of the frequency. This relationship has become known as Moseley’s Law.

Moseley’s Legacy

Moseley’s experiments resoundingly established that the atomic number Z was not just an accident of the order of elements in the periodic table but an essential characteristic of all matter. Furthermore, the conjecture that Z represented the number of positive charges in the nucleus also became firmly established and would lead to the characterisation of the proton a few years afterwards.

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Bohr model of the atom showing the K, L and M electron shells.

Moreover, Moseley’s Law provided independent evidence for the Bohr model of the atom (see diagram) that was being developed in the same year 1913 (excepting that Bohr originally numbered the electron shells from outer to inner whereas Moseley showed that they should be numbered from inner to outer). Not surprisingly, the K-shell of electrons in the diagram is related to the K-series of X-ray spectral lines found in Moseley’s experiments as described above. [See the lecture slides that follow this post for more details.]

Moseley’s findings immediately resolved some long-standing anomalies in Mendeleev’s ordering of the periodic table. For instance, elements 27 and 28, respectively, corresponding to the metals cobalt and nickel, Mendeleev had based the order upon their physical and chemical properties, even though cobalt had a slightly larger atomic weight and technically should have followed nickel. Moseley was able to show that the periodic table follows a rigorously scientific order based upon the atomic number, Z. Because of this, more than a century later, the periodic table has become one of the most indispensable and reliable tools of science.

Moseley’s experiments also revealed that there were gaps at Z-numbers 43, 61, 72, and 75. All these elements were subsequently discovered: two rare naturally occurring elements, with Z-numbers 72 and 75, respectively,  hafnium (in 1923) and rhenium (in 1925). The two radioactive synthetic elements, with Z-numbers of 43 and 61, respectively, were both created in nuclear reactors, technetium (in 1937) and promethium (in 1945).

[It should be noted that Mendeleev also predicted the missing element technetium, some 50 years earlier.]

Modern Applications of Moseley’s Work

The method of X-ray spectrometry that Moseley developed is still being widely used in modern times. It has become known as X-ray Fluorescence Spectrometry (XRF.) have included my lecture slides on XRF from when I was lecturing at Queensland University of Technology (QUT), see below.

_____________

[1] From SBS News, “Key Facts of Gallipoli Casualties,” available online; published 14 April 2014; accessed 11 Nov 2018. Primary source: Australian War Memorial, Bully Beef and Balderdash by Graham Wilson.

[2] Sir Ernest Rutherford, “Moseley’s Work on X-rays,” Nature, 116, 316-7, 1925. This article is freely available online, published 29th August 1925; accessed 11th Nov 2018.

[3] The youngest son of William Henry Bragg, Robert, was also served and was killed during the Gallipoli campaign in September 1915.

[4] A fuller understanding of the presence of Kα and Kβ lines would await the discovery of electron spin and further developments in quantum mechanics.

 

[5] The graph is by Dr Mark Selby using Moseley’s original data published in the Philosophical Magazine and Journal of Science, 26, 1024–1034, 1913.

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