Moseley number

Thus Moseley s series is almost the same as Mendeleev s series of increasing atomic weights. When, however, the elements are arranged, not according to their atomic weights, but according to their atomic numbers (Moseley numbers), the discrepancies between argon and potassium and between iodine and tellurium disappear (10). 

Measurements of the characteristic X-ray line spectra of a number of elements were first reported by H. G. J. Moseley in 1913. He found that the square root of the frequency of the various X-ray lines exhibited a linear relationship with the atomic number of the element emitting the lines. This fundamental Moseley law shows that each element has a characteristic X-ray spectrum and that the wavelengths vary in a regular fiishion form one element to another. The wavelengths decrease as the atomic numbers of the elements increase. In addition to the spectra of pure elements, Moseley obtained the spectrum of brass, which showed strong Cu and weak Zn X-ray lines this was the first XRF analysis. The use of XRF for routine spectrochemical analysis of materials was not carried out, however, until the introduction of modern X-ray equipment in the late 1940s. 

The energy E of the characteristic X-rays within a given series of lines, i.e. Ka, K/J, etc., increases regularly with the atomic number Z. This dependence is called Moseley s law of X-ray emission ... 

I9l 3 H. G. J. Moseley observed regularities in the characteristic X ray spectra of the elements he thereby discovered atomic numbers Z and provided justification for the ordina] sequence of the dements. 

Our present views on the electronic structure of atoms are based on a variety of experimental results and theoretical models which are fully discussed in many elementary texts. In summary, an atom comprises a central, massive, positively charged nucleus surrounded by a more tenuous envelope of negative electrons. The nucleus is composed of neutrons ( n) and protons ([p, i.e. H ) of approximately equal mass tightly bound by the force field of mesons. The number of protons (2) is called the atomic number and this, together with the number of neutrons (A ), gives the atomic mass number of the nuclide (A = N + Z). An element consists of atoms all of which have the same number of protons (2) and this number determines the position of the element in the periodic table (H. G. J. Moseley, 191.3). Isotopes of an element all have the same value of 2 but differ in the number of neutrons in their nuclei. The charge on the electron (e ) is equal in size but opposite in sign to that of the proton and the ratio of their masses is 1/1836.1527. 

The discovery of hafnium was one of chemistry s more controversial episodes. In 1911 G. Urbain, the French chemist and authority on rare earths , claimed to have isolated the element of atomic number 72 from a sample of rare-earth residues, and named it celtium. With hindsight, and more especially with an understanding of the consequences of H. G. J. Moseley s and N. Bohr s work on atomic structure, it now seems very unlikely that element 72 could have been found in the necessary concentrations along with rare earths. But this knowledge was lacking in the early part of the century and, indeed, in 1922 Urbain and A. Dauvillier claimed to have X-ray evidence to support the discovery. However, by that time Niels Bohr had developed his atomic theory and so was confident that element 72 would be a... 

Moseley photographed characteristic spectra for some 38 elements that could serve as x-ray tube targets. In two papers,37 he not only uncovered structure in the K and L spectra—he alscr established the atomic number as more fundamental than the atomic weight, and he provided brilliant support for- the Bohr theory of atomic structure. 

In Figure 1-16, Moseley s data show that atomic number is clearly preferable, as a fundamental quantity, to atomic weight. The linear relationship between the frequency (reciprocal wavelength) v of the Ka line for element of atomic number Z is... 

Fig. 1-16. Moseley plot for Ka2 lines. The curvature at high Z is due to a change in the effective nuclear charge (Z — 1). The insert shows the atomic number Z to be more fundamental than the atomic weight M. X-rays made possible the first experimental determinations of Z. Crosses = atomic weight dots = atomic number.

Ascites tumor cells, determination of dry weight of individual, 299, 300 Atomic absorption coefficient, 15 Atomic energy program, use of x-ray absorptiometry in, 96 Atomic number, significance proved by Moseley, 28, 29... 

The number of protons in an element s atomic nucleus is called the atomic number, Z, of that element. For example, hydrogen has Z = 1 and so we know that the nucleus of a hydrogen atom has one proton helium has Z = 2, and so its nucleus contains two protons. Henry Moseley, a young British scientist, was the first to determine atomic numbers unambiguously, shortly before he was killed in action in World War I. Moseley knew that when elements are bombarded with rapidly moving electrons they emit x-rays. He found that the properties of the x-rays emitted by an element depend on its atomic number and, by studying the x-rays of many elements, he was able to determine the values of Z for them. Scientists have since determined the atomic numbers of all the known elements (see the list of elements inside the back cover). 

One problem with Mendeleev s table was that some elements seemed to be out of place. For example, when argon was isolated, it did not seem to have the correct mass for its location. Its relative atomic mass of 40 is the same as that of calcium, but argon is an inert gas and calcium a reactive metal. Such anomalies led scientists to question the use of relative atomic mass as the basis for organizing the elements. When Henry Moseley examined x-ray spectra of the elements in the early twentieth century, he realized that he could infer the atomic number itself. It was soon discovered that elements fall into the uniformly repeating pattern of the periodic table if they are organized according to atomic number, rather than atomic mass. 

Bohr s model of the hydrogen atom. Identification of atomic number with nuclear charge number (H. Moseley). 

The Moseley equation, v = A(Z -B)2, where v is the frequency of the emitted X-ray radiation, Z is the atomic number and A and B are constants, relates the frequency of emitted X-rays to the nuclear charge for the atoms that make up the target of the cathode ray tube. X-rays are emitted by the element after one of its K-level electrons has been knocked out of the atom by collision with a fast moving electron. In this question, we have been asked to determine the values for the constants A and B. The simplest way to find these values is to plot Vv vs. Z. This plot provides Va as the slope and - Va (B) as the y -intercept. Starting with v = A(Z-B)2, we first take the square root of both sides. This affords Vv = Va (Z - B). Multiplying out this expression gives Vv = Va (Z)... 

The characteristic X-ray wavelengths are tabulated in all standard texts on X-ray spectrometry, but can also be calculated from the atomic number of the element by Moseley s law ... 

Only a few relevant points about the atomic structures are summarized in the following. Table 4.1 collects basic data about the fundamental physical constants of the atomic constituents. Neutrons (Jn) and protons (ip), tightly bound in the nucleus, have nearly equal masses. The number of protons, that is the atomic number (Z), defines the electric charge of the nucleus. The number of neutrons (N), together with that of protons (A = N + Z) represents the atomic mass number of the species (of the nuclide). An element consists of all the atoms having the same value of Z, that is, the same position in the Periodic Table (Moseley 1913). The different isotopes of an element have the same value of Z but differ in the number of neutrons in their nuclei and therefore in their atomic masses. In a neutral atom the electronic envelope contains Z electrons. The charge of an electron (e ) is equal in size but of opposite sign to that of a proton (the mass ratio, mfmp) is about 1/1836.1527). 

It is noted without further comment that Moseley was able to characterize his observed x-ray spectra in such a way that the individual elements are identified purely by their respective atomic numbers without any use of an atomic model. Note again, however, that the denominators in the Qk and Ql expressions are exactly the frequencies of the hydrogenic Lymann and Balmer alpha lines (Moseley s nomenclature) and that therefore Qk and Ql exactly equal the nuclear charge of a one-electron hydrogenic atom which would be deduced from the frequency v of its observed Lymann and Balmer alpha lines2. 

While we have not carefully studied all of the tabular material in Moseley s papers, we note that Moseley s atomic numbers agree with the values accepted today, with one exception. Thus, Moseley assigns N = 66 to Ho and N = 67 to Dy the presently accepted assignment reverses these two elements in the periodic table. 

Moseley s law spect The law that the square-root of the frequency of an x-ray spectral line belonging to a particular series is proportional to the difference between the atomic number and a constant which depends only on the series. mOz-lez, 10 Mossbauer spectroscopy spect The study of Mossbauer spectra, for example, for nuclear hyperfine structure, chemical shifts, and chemical analysis. mus,bau-3r spek tras ko pe ... 

Qualitative analysis is manifested in the identification of the elements present. It is based on Moseley s law, which points out that the energies of a pre-selected line-type (e.g. Kai) lie on a monotonic, smooth curve as a function of the atomic number. Simultaneous read-out of the positions of the many lines present in the EDS spectrum acts as identifying fingerprints and results in a list of the element present in the excited volume. 

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

Additional experimental evidence for the preservation of electronic states upon formation of molecules is the insensitivity of X-ray spectra to the chemical enviromnent of an atom, implying that the ionization potentials of inner electrons are relatively unaffected by bonding. Indeed, Moseley s classic experiments on the relationship between X-ray frequencies and atomic number were carried out on atoms in a variety of states of chemical combination. 

Courtesy of Lyman C. Newell Henry Gwyn Jeffreys Moseley, 1887-1915. English physicist whc studied the X-ray spectra of more than fifty elements and discovered the relation existing between the atomic number of an element and the frequency of the X-rays which it emits when bombarded by cathode rays. At the age of twenty-seven years he was killed while in active service at the Dardanelles. 

When H. G. ]. Moseley discovered the simple relationship which exists between the X-ray spectrum of an element and its atomic number, there were seven unfilled spaces in the periodic table. Elements 43, 61, 72, 75, 85, 87, and 91, were yet to be revealed. Element 91 (protactinium) was discussed with the radioactive elements in Chapter 29. In 1923 D. Coster and G. von Hevesy showed that element 72, hafnium, is widely distributed but that it had escaped detection because of its close resemblance to zirconium. Element 75 (rhenium) was announced by W. and l. Noddack in 1925, and is now a commercial article. 

Moseley stated that, within the limits of his researches, which covered all the elements between aluminum (number 13) and gold (number 79), there were spaces for three missing ones numbers 43, 61, and 75, and that, sinoe their X-ray spectra can be accurately predicted, it ought to be rather easy to find them. It was then believed that the celtium whose arc spectrum Professor Urbain had described in 1911 was element 72 (6, 13,14). 

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