Atomic weight values

Accurate atomic weight values do not automatically follow from precise measurements of relative atomic masses, however, since the relative abundance of the various isotopes must also be determined. That this can be a limiting factor is readily seen from Table 1.3 the value for praseodymium (which has only 1 stable naturally occurring isotope) has two more significant figures than the value for the neighbouring element cerium which has 4 such isotopes. In the twelve years since the first edition of this book was published the atomic weight values of no fewer than 55 elements have been improved, sometimes spectacularly, e.g. Ni from 58.69( 1) to 58.6934(2). 

Table 1.3 Kvolution of atomic weight values for selected elements (the dates selected were chosen for the reasons given below)...

He also points out several times the important evidential role played by the success of Mendeleev s contrapredictions —the corrections of atomic weight values previously assigned to already known elements—such as beryllium, uranium and tellurium—so that they fitted into his table smoothly. And indeed the most elaborate statement of Brush s general conclusion seems to be the following ... 

The basis for the claim of discovery of an element has varied over the centuries. The method of discovery of the chemical elements in the late eightenth and the early nineteenth centuries used the properties of the new sustances, their separability, the colors of their compounds, the shapes of their crystals and their reactivity to determine the existence of new elements. In those early days, atomic weight values were not available, and there was no spectral analysis that would later be supplied by arc, spark, absorption, phosphorescent or x-ray spectra. Also in those days, there were many claims, e.g., the discovery of certain rare earth elements of the lanthanide series, which involved the discovery of a mineral ore, from which an element was later extracted. The honor of discovery has often been accorded not to the person who first isolated the element but to the person who discovered the original mineral itself, even when the ore was impure and that ore actually contained many elements. The reason for this is that in the case of these rare earth elements, the earth now refers to oxides of a metal not to the metal itself This fact was not realized at the time of their discovery, until the English chemist Humphry Davy showed that earths were compounds of oxygen and metals in 1808. 

At the beginning of the twentieth century, the International Committee on Atomic Weights (ICAW) was formed. Although the ICAW did not set internationally approved names, a name with an atomic weight value in their table lent support for the adoption of that name by the chemical community. Twenty years later, the ICAW became a part of the International Union of Pure and Applied Chemistry (lUPAC) when it was formed. lUPAC was called the International Union of Chemistry in those early days. In 1949, the responsibility for acceptance of the name of a chemical element was given by lUPAC to its Commission on Nomenclature of Inorganic Chemistry (CNIC). 

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. 

The number in parentheses following the atomic weight value gives the uncertainty in the last digit. An... 

We find that our appioxunak atomic weight value yields a formula of M20,. which we know cannot be correct We also know that M20O29 is unreasonable What we perceive is that the formula is undoubtedly M2Ot anc tjjat tjje apparent error is undoubtedly caused by the approximate atomic weight Assuming that... 

The following atomic weight values have been used in calculating the atomic weights in this seotion —... 

For amount-of-substance measurements we include kilogram mass units, which are linked to the amount-of-substance unit in SI by the atomic-weight values. The latter differ greatly in uncertainty for different chemical entities, but are always available, with the best estimates by current knowledge of their uncertainties,... 

Atomic weight values. These are fundamental constants available from IUPAC tables... 

The atomic weights given by the first two ratios should be increased by 0-015, to allow for the presence of the trace of sodium bromide present. The third ratio requires no such correction as the weights of the nickel bromide were corrected before computing the ratio. The corrected atomic weight values are accordingly 58-683, 58-680, and 58-682. 

Better determinations were made by Mallet,10 Troost,11 Diehl,12 Stas,13 and Dittmar,14 whose results are tabulated below. The atomic-weight values are calculated from the data (p. 57). 

Otherwise stated, there exists a relationship between Mendeleev s atomic weight values AW- ) and the corresponding modem values (AW2) ... 

Apparently, Mendeleev believed the valency of lanthanum to be 2 instead of 3. As a consequence, he used the incorrect atomic weight of 94. Application of Eq. (7) to the other six elements with wrong atomic weight values, leads to the following valencies used by Mendeleev ... 

The molecular weights are based on the atomic weight values in Atomic weights of the Elements 2001, PURE Appl Chem., 75, 1107, 2003. The densities are given for the temperature indicated and... 

Table 1. Dalton s first set of atomic weight values (1805).

In the 1860s, for instance, the Belgian chemist Jean Servais Stas (1813-91) determined atomic weights more accurately than Berzelius had done. Then, at the begiiming of the twentieth century, the American chemist Theodore William Richards (1868-1928), taking fantastic precautions, produced atomic weight values that may represent the ultimate accuracy possible to purely chemical methods. 

Equally precise and considerably more accurate atomic weight values may be obtained if the potassium permanganate solution is standardized against the oxalate of that rare earth element which is the principal constituent of the mixture. The explanation of the phenomenon may be that the precipitated rare earth oxalates are, in part. 

Previously, the differences in atomic weight values between lanthanum, cerium and didymium had been too small to differentiate between these three elements. This undermined Mendeleev s use of the atomic weight as the characteristic property of basis substances, and led Mendeleev to believing in physically real atoms and an internal matter constituting these atoms. 

Following the adventurous story of terbium it is actually impossible to decide by now who was the true discoverer, Mosander, Delafontaine or Smith The element names, as indicated above were applied inconsistently, and we cannot know whether they referred to the same substance. Did Mosander find the same substance and called it erbium that finally became terbium with Delafontaine, or was Bunsen correct and consequently Mosander s fraction was a mixture only No data were reported that would allow us to state now, at this late date, what substances were identical, no characteristic spectral lines, no exact atomic weight values are at our disposal as yet. 

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