Nucleidic and Atomic Masses

The accepted unit of mass for nucleides is the dalton, symbol d. The dalton is defined as exactly tV of the mass of the neutral atom iCfi. The nucleidic mass of C is exactly 12.00000 d. The dalton is approximately 1.66033 X 10-- kg. 

In a quantitative discussion of an ordinary chemical reaction the relative amounts of various substances that react or are formed can be calcu-., lated from the numbers of atoms of different elements and the masses of the atoms. If an element is present as a mixture of isotopes, it is the average atomic mass that is needed. It is this average atomic mass for an element that is called the chemical atomic mass. The unit for atomic mass is the dalton. 

The name atomic mass is here used in place of atomic weight. There might, however, be some advantage to retaining the old name atomic weight in referring to the natural mixtures of isotopes that are involved in most chemical reactions, and it seems likely that this usage will continue for some time. 

John Dalton chose the value 1 for hydrogen as the base of his scale of atomic masses. The Swedish chemist J. J. Berzelius used 100 for oxygen, and the Belgian chemist J. S. Stas (1813-1891), who carried out many quantitative analyses of compounds, proposed 16 for oxygen (the natural mixture of isotopes), and this base was used for many years. For several decades nucleidic masses were expressed on a scale (called the physical scale) based on iVth the mass of the neutral atom O the chemical atomic-weight unit was then 1.000272 times the physical atomic-mass unit. This period of confusion was brought to an end in 1961 by the acceptance of iVth the mass of -C as the unit for both atomic masses and nucleidic masses. 

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