Atomic weights inversions

Some of the important properties of the elements are given in Table 18.1. The imprecision of the atomic weights of Kr and Xe reflects the natural occurrence of several isotopes of these elements. For He, however, and to a lesser extent Ar, a single isotope predominates ( He, 99.999 863% " Ar, 99.600%) and much greater precision is possible. The natural preponderance of " Ar is indeed responsible for the well-known inversion of atomic weight order of Ar and K in the periodic table, and the position of Ar in front of K was only finally accepted when it was shown that the atomic weight of He placed it in front of Li. The second isotope of helium, He, has only been available in significant amounts since... 

The atomic weight increases regularly across the row except for the inversion at cobalt and nickel. We would expect the atomic weight of Ni to be higher than that of Co because there are more protons (28) in the Ni nucleus than in the Co nucleus (27). The reason for the inversion lies in the distribution of naturally occurring isotopes. Natural cobalt consists entirely of the isotope 2 Co natural nickel consists primarily of the isotopes Ni and Ni, the 58-isotope being about three times as abundant as the 60-isotope. 

Cavendish found that at 30 inches barometer, and 50° Fahrenheit temperature, one ounce of iron gave 412 and one ounce of zinc gave 202 ounce measures. These volumes are approximately inversely proportional to the present atomic weights of these metals.18... 

DU EON C AND PETIT LAW OF SPECIFIC HEATS. Ii has long been known Ural the atomic heals of Ihe great majority of elements have nearly the same value at room temperature in fact, the thermal capacity of a gram-atom of most elements is not far from 6 calories per degree. Dulong and Pelit expressed this by stating that the specific heats of elements arc in inverse proportion to their atomic weights. 

To separate isotopes by this process, they must be in the gaseous form. Therefore, the separation of isolopes of uranium required the conversion of die metallic uranium into a gaseous compound, for which purpose the hexafluoride. UF. was chosen. Since the atomic weight of fluorine is 19, die molecular weight of the hexafluoride of 235 LI is 235 + (6 19) = 349, and the molecular weight of die hexafluoride of 23SU is 238 + (6 x 19) = 352. Since the rate of diffusion of a gas is inversely proportional to die square root of its density (mass per unit volume), the maximum separation factor for one diffusion process of the uranium isotopes is V352/349 = 1.0043, Since only part of the gas can be allowed to diffuse, the, actual separation factoi is even less dian this theoretical maximum. 

The diffusion method depends on the fact that the velocity of a molecule is inversely proportional to the square root of its molecular weight. Slight separations were made years ago in the case of hydrochloric acid and of mercury but the results were disappointing on account of the very small yields. The isotopes of neon with atomic weights of 20 and 22 have been separated rather successfully by diffusion. Hydrogen and deuterium also have been separated by diffusion. 

In the least squares analyses of eq. 3, the individual enthalpies were weighted inversely as the squares of the experimental uncertainty intervals. In all cases, r2 > 0.9999. The standard errors were generated from the unweighted enthalpies. nc is the number of carbon atoms in the compound. 

By measuring the deflection of the beams, relative values of ne/M for different ions can be determined. Since e is constant, relative values of ne/M for different ions are also inverse relative values of M/n therefore this method permits the direct experimental determination of the relative masses of atoms, and hence of their atomic weights. By this method Thomson discovered the first known non-radioactive isotopes, those of neon, in 1913. 

However, the truly toxic action varies inversely as the atomic weight of the halogen. Bromoacetone is less toxic than chloroacetone ... 

Fortunately, there were other keys to correctness in atomic weights. In 1818, for instance, a French chemist, Pierre Louis Dulong (1785-1838), and a French physicist, Alexis Therese Petit (1791-1820), working in collaboration, found one of them. They discovered that the specific heat of elements (the temperature rise that follows upon the absorption of a fixed quantity of heat) seemed to vary inversely with the atomic weight. That is, if element x had twice the atomic weight of element y, the temperature of element x would rise by only half as many... 

Notice that the atomic weight sequence is violated two more times by the inversion of tellurium (Te) and iodine (I), and by the insertion of gold (An) and bismuth (Bi). 

Thomas Graham determined that the rates of diffusion and effusion of gases are inversely proportional to the square roots of their molecular or atomic weights. This is Graham s Law. In general, it says that the lighter the gas, the faster it will effuse (or diffuse). Mathematically, Graham s Law looks like this ... 

Dulong and Petit, law of n. The specific heats of the several elements are inversely proportional to their atomic weights. The atomic heats of solid elements are constant and approximately equal to 6.3. Certain elements of low atomic weight and high... 

By closely studying the work of Kirchoff and Bunsen, Hinrichs found that some of the spectral hne frequencies, those referred to as dark hnes, could be related to the chemistry of the elements through their atomic weights, as well as to their postulated atomic dimensions. The difference between the spectral line frequencies seemed to be inversely proportional to the atomic weights of the elements in question. Hinrichs quoted the values of calcium, where the frequency difference is 4.8 units, and barium, which is chemically similar but has a higher atomic weight and shows a frequency difference of 4.4 units." ... 

---