Atomic weights history

Atomic Weight Determinations. Dictionary of American History, Rev. Edn., Charles Scribner s Sons, New York, 1976, pp. 218-19. 

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. 

As mentioned in the previous section titled A Short History of Chemistry, many scientists identified elements, determined their characteristics, similarities, and differences, and designed symbols for them. Using unique experiments, scientists devised ways to define the structure of atoms and determine atomic weights, sizes, and electrical charges as well as energy levels for atoms. 

Only after the atomic weight units of composition were established, was it possible for chemists to arrange those units successfully in meaningful molecular arrays and conceive the ideas of valence and directional bonding. Molecular structure remains today the central theoretical focus of attention, whether in the realm of molecular biology, biochemistry, chemical pharmaceutics, or polymer plastics. Daltons atomic theory must be seen as the climax of the history of chemical composition and terminates this story. ... 

The analytes and matrices are chosen for their prominence in cross-border trade and environmental or political issues, and they include substances such as rice, wine, water, fuel, and human serum. Table 5.2 is a list of 1MEP rounds up to the end of 2006. The majority of analytes are elements. This reflects the history of 1RMM, which has been prominent in the measurement of atomic weights and the development of IDMS as a primary method for the analysis of elemental amounts. 

Occurrence, Early History, Extraction, Purification - Physical Properties— Chemical Properties--Physiological Action—Atomic Weight- Alloys Doteetion and Estimutlon. 

History—Preparation—Allotropy—Colloidal Arsenio—General Physical Properties of Arsenic—Spectrum—Chemical Properties—Atomic Weight—... 

We will finesse here the long and important struggle of getting a truly self-consistent table of atomic weights. If you are interested in the complex history of this problem and the clear solution to it proposed by S. Cannizzaro in 1860, there are many accounts available in books on the history of chemistry. One example is J. R. Partington, A History of Chemistry, Vol. IV, Macmillan, London, 1964. Relative atomic weights now are based on 12C =12 (exactly). 

Occurrence of Cobalt—History—Preparation—Pyrophoric Cobalt—Physioal Properties—Ooelusion of Hydrogen—Chomioal Properties—Atomic-Weight —Uses -Electro-deposition—Alloys. 

Ooourrenoe of Nickel—History—Preparation in the Laboratory—Commercial Production—Metallurgy of Niokel—Properties—Colloidal Niokel—Atomic Weight—Uses—Eleotro-deposition—Alloys. 

Occurrence and History of Palladium—Preparation—Physical Varieties Physioal Properties—Permeability to Hydrogen--Occlusion of Canos Occlusion of Hydrogen—Chemical Properties--Catalytic Activity Crystalline Palladium—Colloidal Palladium—Spongy Palladium —Palladium Black —Uses—Atomic Weight—Alloys. 

Occurronco and History of Osmium -Preparation -Properties Colloidal Osmium—Catalytic Activity—Atomic Weight—Uses Alloys. 

Occurrouoo and History of Iridium—Preparation—Purilieation -Physical and Chemical Properties—Atomic Weight—Usos—Alloys. 

Occurrence and History of Platinum--Proparatioi) -Punlioulimi Physical Properties—Volatilisation -Diffusion of Cases Solubility of (laMes Chemical Properties - -Catalytic Activity -Passivity Crystalline Platinum —Colloidal Platinum — Platinum Black Platinum Mpongo Explosive Platinum —Atomic Weight — Uses Buhstitutes Alloys Platinum Amalgam. 

---