One of the most important applied aspects of chemical thermodynamics is the question about the laws, explain the existence in nature of different forms of connections: why do some chemical elements occur as sulphides, others form compounds with oxygen, while others are often found in "native", ie. free state? The probability of finding a given element in the "native" state F. A. Letnikov proposed to determine the values of the Gibbs energies of formation of oxygen compounds and sulphides, related to the number of atoms of oxygen or sulfur in the molecule.

The more negative value of the Gibbs energy of formation of compounds, the less likely the finding in nature of this element in "native" condition. Platinum, for example, does not form under natural conditions oxide compounds, but it is found as the sulfide - PtS (the cooperite). The affinity of platinum to sulfur above, than to oxygen. Bismuth, and arsenic, as you know, on the contrary, found in "native" form the sulfides of other elements, because of their extremely low affinity to sulphur. The affinity for oxygen of these elements is significantly higher (cm. table. 2.1.)

Table 2.1.^*

^* The data are taken from the book: Bulakh A. G. “Methods of thermodynamics in Mineralogy”. L., “Nedra”,. 1968 g.