Henry Moseley

Henry Moseley Henry Moseley 5 030811.doc

Henry Moseley (1887-1915): A British chemist who studied under Rutherford and brilliantly developed the application of X-ray spectra to study atomic structure; his discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers. Tragically for the development of science, Moseley was killed in action at Gallipoli in 1915.

In 1913, almost fifty years after Mendeleev, Henry Moseley published the results of his measurements of the wavelengths of the X-ray spectral lines of a number of elements which showed that the ordering of the wavelengths of the X-ray emissions of the elements coincided with the ordering of the elements by atomic number. With the discovery of isotopes of the elements, it became apparent that atomic weight was not the significant player in the periodic law as Mendeleev, Meyers and others had proposed, but rather, the properties of the elements varied periodically with atomic number.

When atoms were arranged according to increasing atomic number, the few problems with Mendeleev's periodic table had disappeared. Because of Moseley's work, the modern periodic table is based on the atomic numbers of the elements. http://www.chemistry.co.nz/henry_moseley.htm

Note: The following comes from John L. Park http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/AtNum-Moseley.html

Leading up to Moseley - Atomic Weights and Periodic Properties

Henry Gwyn Jeffreys Moseley was born on November 23, 1887 and would die in battle on As an aside, he also showed that there are no August 10, 1915, before he turned 28. However, as elements lighter than hydrogen (atomic number = 1) long as our civilization stands, he will be and that there is no possibility for elements between remembered as the man who numbered the elements. hydrogen and helium (atomic number = 2). Both That work, completed in a six-month span during possibilities had been advanced, with some proposals 1913 and 1914 and published in the last two papers demanding three elements between H and He. of his life was a tour de force of scientific accomplishment. Said Robert Millikan: I. The Concept of Atomic Weight

"In a research which is destined to rank as one of Leucippus and Democritus (about 440 BC) are the dozen most brilliant in conception, skillful in credited with the origin of the atom concept. It was execution, and illuminating in results in the Epicurus, slightly more than 100 years later, who history of science, a young man twenty-six years added weight as a property of atoms. old threw open the windows through which we can glimpse the sub-atomic world with a The first tables of relative atomic weights were definiteness and certainty never dreamed of prepared by John Dalton about 1803. There was before. Had the European War had no other much discussion and controversy over the next result than the snuffing out of this young life, that several decades concerning atomic weights. Various alone would make it one of the most hideous and authoritative chemists of the time prepared most irreparable crimes in history." competing tables of atomic weights with many values the same, but a significant number of differences. A brief summary of atomic weights and periodic Some issues were not be fully resolved until 1860, properties is in order. Before Moseley, periodic tables when wide-spread agreement about atomic weight were created on the basis of increasing atomic weight values in the chemistry community started to come (with two exceptions). Moseley showed that the together. correct ordering of the periodic table is on the basis of the atomic number (the number of positive charges in the nucleus). II. Periodic Properties of Elements Mendeleev had periods of eight like Newlands, but he also correctly allowed for longer periods in the Ten elements were known from pre-historical times. transition and rare earth elements. He made a number Phosphorus was discovered about 1665 and from of correct predictions for missing elements and he then, up to 1800, 20 more elements were discovered. had Co/Ni and Te/I in their correct chemical order -- There was an explosion of element discovery starting the reverse of the order based on increasing atomic around 1800, with 27 more elements being weights. He ordered the elements on their atomic discovered by the 1840s. weight except for the two pairs just noted, which he put in their correct chemical order, even though no Starting in 1816, but not fully developed until nearly one knew why. 1830, Johann Wolfgang Döbereiner was the first person to emphasize chemical similarities, pointing It would be Moseley that finally gave the correct out "triads" of elements like lithium, sodium and answer to why the elements were reversed from a potassium as well as chlorine, bromine and iodine. strict ordering based on atomic weights. The He published five triads as well as several elements were correctly ordered based on the atomic "incomplete" triads. numbers.

John Alexander Reina Newlands, working after the The Modern Concept of Atomic Number reform of atomic weights in 1860, was the first to proclaim a pattern for ALL elements. His tables, Henry Gwyn-Jefferies Moseley found that certain done in 1864 and 1865, followed what he called the lines in the X-ray spectrum of each element moved "Law of Octaves." This meant that, when ordered by the same amount each time you increased the atomic atomic weights, every eighth element showed similar number by one. Today, we know that the atomic chemical properties. In his early tables, he left gaps number gives the number of protons (positive for missing elements, but his final table of 1865 left charges) in the nucleus, but the proton had not been no gaps whatsoever. Also, he put two elements into discovered at the time of Moseley’s work. the same position several times. Finally, he allowed for no period longer than eight. Rutherford (in 1914) described Moseley's discovery like this: However, with Newlands, the "atomic number" first "Recently Moseley has supplied very valuable enters the scene. His table of 1865 shows no atomic evidence that this rule [atomic numbers changing weights and simply numbers the elements in order by one from element to element] also holds for a from 1 to 56. number of the lighter elements. By examination of the wave-length of the characteristic X rays Newlands' work was not favorably received. In emitted by twelve elements varying in atomic March, 1866 he spoke on his work and one of his weight between calcium (40) and zinc (65.4), he listeners, a man named Carey Foster with no other has shown that the variation of wave-length can claim to fame, rose to facetiously ask if Newlands be simply explained by supposing that the charge had ever attempted to classify the elements in on the nucleus increases from element to element alphabetical order. by exactly one unit. This holds true for cobalt and nickel, although it has long been known that they III. The Modern Periodic Table occupy an anomalous relative position in the periodic classification of the elements according The modern periodic table was developed to atomic weights." (discovered? invented?) by Dmitri Mendeleev during the years 1869-1871. (Some historians credit others I. Atomic Structure: 1903 - 1911 as co-discoverers, but we will ignore them.) Mendeleev did not use the "atomic number" that Exactly where the positive protons (and the negative Newlands had used. "Atomic number" remained a electrons) were in the atom took time to be worked number without any physical meaning. It was simply out. Keep in mind that the electron (the first sub- the numbering of the elements after they had been atomic particle discovered) was not discovered until placed in order by atomic weight. 1897. (1) J.J. Thomson in 1903, found electrons to be negatively charged particles with mass. He II. Moseley's X-Ray Spectra Work predicted that atoms contained electrons and some kind of positively charged substance, both Moseley's problem was to find a linear relationship of which are evenly distributed throughout the between the atomic number and a measurable atom. property of the nucleus. The atomic number increased by steps of one (18, 19, 20, etc). Moseley (2) In 1911 Rutherford announced his atomic model: needed some function of a nuclear property that (a) a nucleus - a dense concentration of positive increased in the same pattern, that is, by one for each charge with (b) electrons outside the nucleus. element in turn. He found it in the K line of the X-ray spectra of each element. He found that the square (3) In 1913, Bohr took up the question of electrons root of the frequency increases by a constant value and the hydrogen spectrum, and Moseley took up for each integer increase in the atomic number. the investigation of atomic number. He may have choosen to study this area because of Moseley was part of Rutherford's research group -- the work of Charles Barkla, who had demonstrated having arrived in Manchester just weeks before that the elements emitted characteristic X-rays, Rutherford published his great nucleus paper -- when called K and L rays. These X-rays were independent he started his atomic number work. Rutherford was of the physical or chemical state of the element. not all that excited by Moseley wanting to study X- Moseley realized that this meant the X-rays were rays, but the energy and enthusiasm of the younger characteristic of the nucleus. man soon wore Rutherford down. Moseley determined the wavelengths of the K [You might notice that neutrons have not been radiation using techniques discovered by the father- mentioned. It would not be until 1920 that Rutherford and-son team of W.L Bragg and W.H. Bragg. proposed the existence of a neutral particle -- the Moseley was confident that all he could find a linear neutron. Another of Rutherford's students -- James relationship, but getting the equipment to work Chadwick -- won the 1935 Nobel Prize for reliably was the most time-consuming part of the discovering the neutron in 1932.] entire research project.

Within a few months of Rutherford's nucleus paper Moseley found a linear relationship between the being published, the true, physical meaning of square root of the frequency and atomic number., as "atomic number" was suggested by A. van den Broek. In 1913, he wrote:

"In a previous letter to NATURE (July 20, 1911, p. 78) the hypothesis was proposed that the atomic weight being equal to about twice the intra-atomic charge, 'to each possible intra- atomic charge corresponds a possible element,' or that (Physik. Zeitschr, xiv., 1912, p. 39), 'if all elements be arranged in order of increasing atomic weights, the number of each element in that series must be equal to its intra-atomic charge.' "

shown below. Notice that the list follows the chemical order of Co More on X-Ray Spectra then Ni. If the list were ordered by strict atomic weights, the Ni would come first. A brief summary of X-ray research is in order, since Moseley will use a regular change in the position of Since no one could yet measure the wavelengths (or lines in the X-ray spectrum of each element to assign frequencies) of X-rays, Barka measured the a positive charge (the atomic number) to the nucleus absorbance of each secondary radiation. He did so by of each element. directing the secondary radiation through a 0.01 cm layer of aluminum and measuring how much of the I. The Discovery of Secondary X-Rays beam was absorbed.

Our X-ray thread starts in the evening of November It turns out that the "hard" radiation (the more 8, 1895. This is the day that Wilhelm Conrad penetrating ones) has the shortest wavelength (which Röntgen discovered X-rays. He realized the also means the highest frequency and the highest importance of his discovery at once. He stayed up all energy). So as the atomic weight increased (with the night doing experiments and even ate and slept in the Co/Ni exception), the secondary X-ray became laboratory for a time. His "preliminary harder and harder. "Soft" X-rays means of lower communication" on X-rays was turned in on penetrating ability, so much of the secondary beam is December 28, 1895 and published before the end of absorbed. (Soft means longer wavelength X-rays the year. Very quickly, others began studying X-rays, which also means lower frequency and lower with many new discoveries being made. energy.) In the early years, elements below about aluminum could not be studied due to the instruments For us, the next step in our story was made in 1897. It not being sensitive enough to measure the X-rays was found that, when a primary X-ray beam was after absorption. directed at a substance, that substance gave off secondary X-rays. (Please realize that many other III. A Second X-Radiation is Found discoveries were made about X-rays. I'm just highlighting the ones which culminate in Moseley's Barkla (and his students) continued the detailed study work.) of secondary X-radiation. In 1909, Barkla published another paper in which he found that the supposedly II. Secondary X-Rays are Characteristic of the homogeneous secondary X-rays were, in fact, Element heterogeneous. He wrote:

The next discovery was made by Charles G. Barkla. "The writer has recently investigated more He found a connection between the atomic weight of closely the radiations from Sn, Sb, I (which have the element and its secondary X-rays. His first efforts been recorded as elements emitting a radiation of in this area were in 1906 (the same year Rutherford variable penetrating power). It has been found discovered alpha-particle scattering) and in 1909 he that these consist of a very easy absorbed wrote: radiation and a very penetrating homogeneous radiation superposed. The absorptions of the "It has been found that each of the elements Cr, penetrating portions of the beams from each Fe, Co, Ni, Cu, Zn, As, Se, Ag, when subject to a element are shown in fig. I on curve B. The suitable primary beam of X-rays, emits an almost percentage absorptions of the soft radiations from perfectly homogeneous beam of X-rays, the these elements have not yet been determined, but penetrating power of which is characteristic of they are roughly indicated on curve A in fig. 1. the element emitting it." Though a full analysis of the radiations from W, Pt, Au, Pb, Bi, etc., has not yet been made, there Barkla ordered the list of elements above by the is strong evidence that the observed radiations penetrating power of the secondary radiation with the from these elements are also principally Cr called "soft," which means not very penetrating, homogeneous radiations characteristic of the up to silver which is very penetrating or "hard." 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 wrote this slightly two years before Moseley would publish his historic papers in December 1913 and April 1914. 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 wavelength 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."

Some material was edited by MJ This material is copyrighted by John L. Park, and can be found at http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/AtNum-Moseley.html