Chemical scale of atomic weight

The values of the fundamental physical constants have undergone revision in recent years, and cannot be said to be satisfactorily established at present. The following table gives three sets of figures, proposed by Birge (1941), Kaye and Laby (1948), and Du Mond and Cohen (1948). All values are on die chemical scale of atomic weights 0=16-0000. 

Atoms are small pieces of matter, so they have mass. As noted in Section 2.1, a key postulate of Dalton s atomic theory is that mass is conserved during chemical reactions. Much of what we know about chemical reactions and the behavior of substances, therefore, has been derived by accurate measurements of the masses of atoms and molecules (and macroscopic collections of atoms and molecules) that are undergoing change. Chances are that you are already using mass measurements in the laboratory portion of your course in order to monitor changes that occur in chemical reactions. In this section we will discuss the mass scale that is used for atoms and introduce the concept of atomic weights. In Section 3.3 we will extend these concepts to show how fliese atomic masses are used to determine the masses of compounds and molecular weights. 

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. 

Phosphorus has only one stable isotope, J P, and accordingly (p. 17) its atomic weight is known with extreme accuracy, 30.973 762(4). Sixteen radioactive isotopes are known, of which P is by far the most important il is made on the multikilogram scale by the neutron irradiation of S(n,p) or P(n,y) in a nuclear reactor, and is a pure -emitter of half life 14.26 days, 1.7()9MeV, rntan 0.69MeV. It finds extensive use in tracer and mechanistic studies. The stable isotope has a nuclear spin quantum number of and this is much used in nmr spectroscopy. Chemical shifts and coupling constants can both be used diagnostically to determine structural information. 

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). 

All elements, by definition, have a unique proton number, but some also have a unique number of neutrons (at least, in naturally occurring forms) and therefore a unique atomic weight - examples are gold (Au Z = 79, N = 118, giving A =197), bismuth (Bi Z = 83, N = 126, A = 209), and at the lighter end of the scale, fluorine (F Z = 9, N = 10, A = 19) and sodium (Na Z = 11, N= 12, A = 23). Such behavior is, however, rare in the periodic table, where the vast majority of natural stable elements can exist with two or more different neutron numbers in their nucleus. These are termed isotopes. Isotopes of the same element have the same number of protons in their nucleus (and hence orbital electrons, and hence chemical properties), but... 

Natural carbon consists of a 99/1 ratio of C12 to C13. The Masses of these two isotopes on the conventional physical scale are 12.0036 and 13.0073. Show that a physical scale based on C12 = 12.0000 would result in an atomic weight of naturally occurring carbon very nearly the same as that on the present chemical scale (12.011). 

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