Atomic weight Scale

Dual atomic-weight scales based on oxygen = 16 (chemical) and = 16 (physical) abandoned in favour of the present unified scale based on C = 12. 

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). 

This average weight is the weight that all carbon atoms appear to have as they are dealt with by chemists. The average atomic weights for all the elements have been determined in a similar way, and it is these averages that are listed in the relative atomic weight scale inside the back cover. 

Atoms are so tiny that, until recently, the masses of individual atoms could not be measured directly (Figure 3.7). However, because mass was so important in Dalton s theory, some measure of atomic masses was necessary. Therefore, a relative scale—the atomic mass scale—is used. This scale is sometimes called the atomic weight scale. On this scale, an average of the masses of all the atoms of the naturally occurring mixture of isotopes of a given element is measured relative to the mass of an atom of a standard. 

The History of the Atomic Weight Scale. John Dalton (1776-1844), who in 1803 made the old atomic hypothesis into a useful scientific theory by developing... 

The isotopes of the elements show several interesting properties. The known isotopes for the first 10 elements are listed in Table 33-2. The masses given in column 3 of this table refer to the physicists atomic weight scale, in which O = 16.00000. 

We said in Section 2-5 that the atomic weight scale is based on the mass of the carbon-12 isotope. As a result of action taken by the International Union of Pure and Applied Chemistry in 1962,... 

Prior to 1962, the atomic weight scale was based on the assignment of an atomic weight of exactly 16 amu to the naturally occurring mixmre of oxygen. The atomic weight of cobalt is 58.9332 amu on the carbon-12 scale. What was it on the older scale ... 

Dalton then proceeded to develop a relative atomic weight scale, taking the atomic weight of hydrogen as his unit. He assumed that all atoms of the same element had identical atomic weights and that the weights of atoms of different elements differed. Chemical combination occurred between small numbers of atoms of two or more elements. Finally, he believed that in most circumstances the most common compound of two elements involved combination of one atom from each element to form the smallest particle of the compound. 

Continuity in the atomic weight scale and discontinuity in the chain of beings (A/N)... 

Problem 7-1. Calculate the frequencies and wave lengths of the first five members of the Balmer scries for the isotopic hydrogen atom whose mass is approximately 2.0136 on the atomic weight scale, and compare with those for ordinary hydrogen. 

We begin by obtaining a conversion factor from atomic nuclear energy to mass on the conventional atomic weight scale . Denoting the faraday by F we have for this factor... 

This is equivalent to a mass on the conventional atomic weight scale... 

Eventually, anyone who wants to carry out a chemical reaction must figure out how much of the reactants must he combined to make the desired amount of product. Somehow a connection must be made between atoms, molecules, and ions at the nanoscopic scale and weighable amounts of chemicals. Here is where balanced equations are essential. The relative atomic weight scale and balanced chemical equations together make it possible to answer How much questions. 

Sometimes the term molecular weight is used instead of molar mass. Both terms refer to the relative mass of a substance according to the atomic weight scale. Strictly speaking, a molecular weight is for one molecule and would be in atomic mass units. We need use only molar masses. 

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