Henry Moseley. Leading up to Moseley - Atomic Weights and Periodic Properties

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1 Henry Moseley Henry Moseley doc Henry Moseley ( ): 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 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. Note: The following comes from John L. Park 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 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 published in the last two papers of his life was a tour de force of scientific accomplishment. Said Robert Millikan: "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 certainty 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 atomic weights and periodic properties is in order. Before Moseley, periodic tables were created on the basis of increasing atomic weight (with two exceptions). Moseley showed that the correct ordering of the periodic table is on the basis of the atomic number (the number of positive charges in the nucleus). As an aside, he also showed that there are no elements lighter than hydrogen (atomic number = 1) and that there is no possibility for elements between hydrogen and helium (atomic number = 2). Both possibilities had been advanced, with some proposals demanding three elements between H and He. I. The Concept of Atomic Weight Leucippus and Democritus (about 440 BC) are credited with the origin of the atom concept. It was Epicurus, slightly more than 100 years later, who added weight as a property of atoms. The first tables of relative atomic weights were prepared by John Dalton about There was much discussion and controversy over the next several decades concerning atomic weights. Various authoritative chemists of the time prepared competing tables of atomic weights with many values the same, but a significant number of differences. Some issues were not be fully resolved until 1860, when wide-spread agreement about atomic weight values in the chemistry community started to come together.

2 II. Periodic Properties of Elements Ten elements were known from pre-historical times. Phosphorus was discovered about 1665 and from then, up to 1800, 20 more elements were discovered. There was an explosion of element discovery starting around 1800, with 27 more elements being discovered by the 1840s. Starting in 1816, but not fully developed until nearly 1830, Johann Wolfgang Döbereiner was the first person to emphasize chemical similarities, pointing out "triads" of elements like lithium, sodium and potassium as well as chlorine, bromine and iodine. He published five triads as well as several "incomplete" triads. John Alexander Reina Newlands, working after the reform of atomic weights in 1860, was the first to proclaim a pattern for ALL elements. His tables, done in 1864 and 1865, followed what he called the "Law of Octaves." This meant that, when ordered by atomic weights, every eighth element showed similar chemical properties. In his early tables, he left gaps for missing elements, but his final table of 1865 left no gaps whatsoever. Also, he put two elements into the same position several times. Finally, he allowed for no period longer than eight. However, with Newlands, the "atomic number" first enters the scene. His table of 1865 shows no atomic weights and simply numbers the elements in order from 1 to 56. Newlands' work was not favorably received. In March, 1866 he spoke on his work and one of his listeners, a man named Carey Foster with no other claim to fame, rose to facetiously ask if Newlands had ever attempted to classify the elements in alphabetical order. III. The Modern Periodic Table The modern periodic table was developed (discovered? invented?) by Dmitri Mendeleev during the years (Some historians credit others as co-discoverers, but we will ignore them.) Mendeleev did not use the "atomic number" that Newlands had used. "Atomic number" remained a number without any physical meaning. It was simply the numbering of the elements after they had been placed in order by atomic weight. Mendeleev had periods of eight like Newlands, but he also correctly allowed for longer periods in the transition and rare earth elements. He made a number of correct predictions for missing elements and he had Co/Ni and Te/I in their correct chemical order -- the reverse of the order based on increasing atomic weights. He ordered the elements on their atomic weight except for the two pairs just noted, which he put in their correct chemical order, even though no one knew why. It would be Moseley that finally gave the correct answer to why the elements were reversed from a strict ordering based on atomic weights. The elements were correctly ordered based on the atomic numbers. The Modern Concept of Atomic Number Henry Gwyn-Jefferies Moseley found that certain lines in the X-ray spectrum of each element moved the same amount each time you increased the atomic number by one. Today, we know that the atomic number gives the number of protons (positive charges) in the nucleus, but the proton had not been discovered at the time of Moseley s work. Rutherford (in 1914) described Moseley's discovery like this: "Recently Moseley has supplied very valuable evidence that this rule [atomic numbers changing by one from element to element] also holds for a number of the lighter elements. By examination of the wave-length of the characteristic X rays emitted by twelve elements varying in atomic weight between calcium (40) and zinc (65.4), he has shown that the variation of wave-length can be simply explained by supposing that the charge on the nucleus increases from element to element by exactly one unit. This holds true for cobalt and nickel, although it has long been known that they occupy an anomalous relative position in the periodic classification of the elements according to atomic weights." I. Atomic Structure: Exactly where the positive protons (and the negative electrons) were in the atom took time to be worked out. Keep in mind that the electron (the first subatomic particle discovered) was not discovered until 1897.

3 (1) J.J. Thomson in 1903, found electrons to be negatively charged particles with mass. He predicted that atoms contained electrons and some kind of positively charged substance, both of which are evenly distributed throughout the atom. (2) In 1911 Rutherford announced his atomic model: (a) a nucleus - a dense concentration of positive charge with (b) electrons outside the nucleus. (3) In 1913, Bohr took up the question of electrons and the hydrogen spectrum, and Moseley took up the investigation of atomic number. Moseley was part of Rutherford's research group -- having arrived in Manchester just weeks before Rutherford published his great nucleus paper -- when he started his atomic number work. Rutherford was not all that excited by Moseley wanting to study X- rays, but the energy and enthusiasm of the younger man soon wore Rutherford down. [You might notice that neutrons have not been mentioned. It would not be until 1920 that Rutherford proposed the existence of a neutral particle -- the neutron. Another of Rutherford's students -- James Chadwick -- won the 1935 Nobel Prize for discovering the neutron in 1932.] Within a few months of Rutherford's nucleus paper being published, the true, physical meaning of "atomic number" was suggested by A. van den Broek. In 1913, he wrote: II. Moseley's X-Ray Spectra Work Moseley's problem was to find a linear relationship between the atomic number and a measurable property of the nucleus. The atomic number increased by steps of one (18, 19, 20, etc). Moseley needed some function of a nuclear property that increased in the same pattern, that is, by one for each element in turn. He found it in the K line of the X-ray spectra of each element. He found that the square root of the frequency increases by a constant value for each integer increase in the atomic number. He may have choosen to study this area because of the work of Charles Barkla, who had demonstrated that the elements emitted characteristic X-rays, called K and L rays. These X-rays were independent of the physical or chemical state of the element. Moseley realized that this meant the X-rays were characteristic of the nucleus. Moseley determined the wavelengths of the K radiation using techniques discovered by the fatherand-son team of W.L Bragg and W.H. Bragg. Moseley was confident that all he could find a linear relationship, but getting the equipment to work reliably was the most time-consuming part of the entire research project. Moseley found a linear relationship between the square root of the frequency and atomic number., as shown below. "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 intraatomic 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 intraatomic charge.' "

4 More on X-Ray Spectra 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, 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 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."

5 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 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