International relative atomic weights

In precise thermodynamic work it must also be noted that relative atomic weights (usually called atomic weights ) are revised every two years (in odd numbered years) by the International Commission on Atomic Weights, so that when values for molar quantities are listed it is essential to record the values of atomic weights used. 

Since 1888 Brauner attempted to put through, jointly with the American chemist F. P. Venable (1856-1934) and other chemists, the atomic weight of oxygen (16) as standard for calculation of relative atomic weights of elements this proposal was only accepted in 1900 at the 4th International Congress of Applied Chemistry in Paris. In the years between 1921 and 1930 Brauner was member of the International Committee on Chemical Elements and president of its subcommission on atomic weights. 

Formula Weights are based on the International Atomic Weights of 1973 and are computed to the nearest hundredth. Density values are given at room temperature unless otherwise indicated by the superscript figure thus 0.9711112 indicates a density of 0.9711 for the substance at 112°C. A densityof 0.89 946 indicates a density of 0.899 for the substance at 16 °C relative to water at 4 °C. 

These atomic weights are those adopted by the International Union of Pure and Applied Chemistry and are based on a relative atomic mass of C12 = 12.000. 

For many years an atomic weight of 44 1 was accepted for scandium, but in 1923 Aston showed that the element had no isotopes and that its atomic mass relative to oxygen 16 was 45. As a result of fresh chemical investigation, the atomic weight 45 1 was accepted by the International Committee in 1925, and this value is accepted to-day (1950). 

After establishment of the fundamental laws of chemistry, units like gram-atom or gram-molecule, were used to specify amounts of chemical elements or compounds. These units are directly related to atomic weights and molecular weights. These units refer to relative masses. The advent of mass spectrometry showed that the atomic weights arise from mixtures of isotopes. Intermittently two scales, a chemical scale and a physical scale were in use. In 1960, by an agreement between the International Union of Pure and Applied Physics (lUPAP) and the International Union of Pure and Applied Chemistry (lUPAC), this duality was eliminated. 

The mass of an atom depends on the number of electrons, protons, and neutrons it contains and all atoms of a given isotope are identical in mass. The SI unit of mass (the kilogram) is too large to function as a convenient unit for the mass of an atom, thus a smaller unit is desirable. In 1961, the International Union of Pure and Applied Chemistry (lUPAC) defined the atomic mass unit (u) to be exactly equal to one-twelfth the mass of one carbon-12 atom. Carbon-12 ( C) is the carbon isotope that has six protons, six neutrons, and six electrons. Using this definition, we have that 1 u = 1.660539 X 10 kg. The atomic mass (sometimes called atomic weight) of an atom is then defined, relative to this standard, as the mass of the atom in atomic mass units (u). For example, the two naturally occurring isotopes of hefium, He and " He, have atomic masses of 3.01602931 u and 4.00260324 u, respectively. This means that a helium-4 (" He) atom is 4.00260324/12 = 0.33355027 times as massive as a carbon-12 atom. ... 

The theoretical chemical formula of a mineral is unique and identifies only one species. Nevertheless, the actual chemical composition is usually variable within a limited range owing to the isomorphic substitutions (i.e., diadochy), or/and low presence of traces of impurities. The relative atomic or molecular mass (based on C = 12.000) of minerals is calculated from the theoretical formula using the last value of atomic masses adopted by the International Union of Pure and Applied Chemistry (lUPAC) in 2001, and the theoretical chemical composition is commonly expressed in percentage by weight (wt.%) of elements and sometimes oxides for oxygenated minerals. 

Atomic mass (atomic weight) Relative mass of an atom. By current international agreement, the standard for all atomic masses is the isotope carbon-12, which is arbitrarily assigned an atomic mass of exactly 12. Atomic mass was the original numerical basis for the periodic table. 

Before the 1970s, two conventions were used for determining relative atomic masses. Physicists related their mass spectrometric determinations to the mass of (i.e. has a mass of exactly 16 on the amu scale), the most abundant isotope of oxygen, and chemists used the weighted mass of all three isotopes of oxygen and At an international congress devoted... 

Coplen, T.B. (2008) Explanatory Glossary of Terms Used in Expression of Relative Isotope Ratios and Gas Ratios, International Union of Pure and Applied Chemistry Inorganic Chemistry Division, Commission on Isotopic Abundances and Atomic Weights, Peer Review, January 16, 2008, http //www.iupac.org (accessed 14 May 2014). 

The effective dose equivalent (M ) is the formulation for the weighted dose equivalents in irradiated tissues or organs stipulated in 1977 by the International Commission on Radiological Protection [ICRP (1977a)]. He is based on an ICRP analysis of the risk information in the 1977 report of the United Nations Scientific Committee on the Effects of Atomic Radiation [UNSCEAR (1977)]. The formulation is given in Table 1.1, where Wi is the weighting factor for the relative radiosensitivity of the tissue and H-y is the dose equivalent in the irradiated tissue or organ. 

Values of atom fraction of deuterium in the aqueous nano-droplet was obtained by weighting, in 1 mL volumetric tubes, the appropriate volumes of matched surfactant stock solutions, one containing solubilized (pure) H2O, the other solubilized (pure) D2O. A Bruker DRX-500 NMR spectrometer was used. The spectra were recorded at 25.0 °C, at a digital resolution of 0.06 Hz/data point. The spectrometer probe temperature was periodically monitored, as recommended elsewhere [21] values of 5obs (after 10 min in the sample compartment for thermal equilibration, measured relative to internal TMS) were within the digital resolution limit. 

---