Standard atomic weight

C = 12 internationally adopted as the unified atomic weight standard by both chemi.sts and physicists. 6-coordinate carbon established in various carboranes by W. N. Lipscomb and others. (Nobel Prize 1976 for structure and bonding of boranes). 

C = 12 internationally adopted as the unified atomic weight standard by both chemi,sts and physicists. 

In 1961, however, international organizations of both chemists and of physicists agreed to adopt an atomic weight standard based on carbon-12 set equal to exactly 12.0000. This new standard was almost exactly that of the old chemical atomic weights and yet it was tied to a single isotope and not to the average of a group of them. 

Its atomic weight was used as a standard of comparison for each of the other elements until 1961 when the International Union of Pure and Applied Chemistry adopted carbon 12 as the new basis. 

Atomic Weights of the Elements Based on the Carbon 12 Standard... 

W, Pt, Au and the noble gases, though 0x0 compounds of all elements are known except for He, Ne, Ar and possibly Kr. This great range of compounds was one of the reasons why Mendeleev chose oxides to exemplify his periodic law (p. 20) and why oxygen was chosen as the standard element for the atomic weight scale in the early days when atomic weights were determined mainly by chemical stoichiometry (p. 16). 

Atomic number Atomic weight Crystal structure Melting Density Thermal Electrical resistivity (at 20°C) Temperature coefficient of resistivity Specific Thermal Standard electrode potential Thermal neutron absorption cross-section. 

The standard for atomic weights is 12C, at exactly 12 amu. What is the standard for formula weights ... 

First and foremost among these was the atomic theory. Throughout the century, chemists were divided about the validity of so-called physical atomism, namely, that there exists a unique indivisible particle specific to each chemical element and characterized by an atomic weight that some chemists sought to prove was a multiple of the standard weight of hydrogen (or its subweight). 

Sheilds WR, Murphy TJ, Catanzaro EJ, Garner J (1966) Absolute isotopic abundance ratios and the atomic weight of a reference sample of chromium. J Res Natl Bur Standards 70A(2) 193-197 Shukolyukov A, Lugmair GW (1998) Isotopic evidence for the Cretaceous-Tertiary impactor and its type. Science 282(5390) 927-929... 

At one time, the hydrogen atom with one proton and no neutron was used as the standard to define 1 atomic mass unit (1 amu). Today, chemists use carbon-12, the most abundant isotope of carbon for the standard amu, which is defined as 1/12 of the C-12 atom. Therefore, the actual atomic weight for an element is in average mass units (numbers), taking into account all the isotopes (atoms) of that element. 

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