Релятивистік механика және кванттық химия: ауыр элементтердің ерекшеліктері

Theories of quantum chemistry explained via relativistic mechanics
Relativistic quantum chemistry combines relativistic mechanics with quantum chemistry to calculate elemental properties and structure, especially for the heavier elements of the periodic table. A prominent example is an explanation for the color of gold: due to relativistic effects, it is not silvery like most other metals. The term relativistic effects was developed in light of the history of quantum mechanics. Initially, quantum mechanics was developed without considering the theory of relativity. Relativistic effects are those discrepancies between values calculated by models that consider relativity and those that do not. Relativistic effects are important for heavier elements with high atomic numbers, such as lanthanides and actinides. Relativistic effects in chemistry can be considered to be perturbations, or small corrections, to the non relativistic theory of chemistry, which is developed from the solutions of the Schrödinger equation. These corrections affect the electrons differently depending on the electron speed compared with the speed of light. Relativistic effects are more prominent in heavy elements because only in these elements do electrons attain sufficient speeds for the elements to have properties that differ from what non relativistic chemistry predicts.

The periodic table was constructed by scientists who noticed periodic trends in known elements of the time. Indeed, the patterns found in it are what give the periodic table its power. Many of the chemical and physical differences between the 5th period (Rb–Xe) and the 6th period (Cs–Rn) arise from the larger relativistic effects of the latter. These relativistic effects are particularly large for gold and its neighbors platinum and mercury. An important quantum relativistic effect is the van der Waals force.

Mercury (Hg) is a liquid down to approximately −39 °C, its melting point. Bonding forces are weaker for Hg–Hg bonds than for their immediate neighbors such as cadmium (m. p. 321 °C) and gold (m. p. 1064 °C). The lanthanide contraction only partially accounts for this anomaly. Au2(g) and Hg(g) are analogous with H2(g) and He(g) with regard to having the same nature of difference. The relativistic contraction of the 6s2 orbital leads to gaseous mercury sometimes being referred to as a pseudo noble gas. Caesium, the heaviest of the alkali metals that can be collected in quantities sufficient for viewing, has a golden hue, whereas the other alkali metals are silver white. However, relativistic effects are not very significant at Z = 55 for caesium (not far from Z = 47 for silver). The golden color of caesium comes from the decreasing frequency of light required to excite electrons of the alkali metals as the group is descended. For lithium through rubidium, this frequency is in the ultraviolet, but for caesium it reaches the blue violet end of the visible spectrum; in other words, the plasmonic frequency of the alkali metals becomes lower from lithium to caesium. Thus caesium transmits and partially absorbs violet light preferentially, while other colors (having lower frequency) are reflected; hence it appears yellowish.

Without relativity, lead (Z = 82) would be expected to behave much like tin (Z = 50), so tin–acid batteries should work just as well as the lead–acid batteries commonly used in cars. However, calculations show that about 10 V of the 12 V produced by a 6 cell lead–acid battery arises purely from relativistic effects, explaining why tin–acid batteries do not work.

In Tl(I) (thallium), Pb(II) (lead), and Bi(III) (bismuth) complexes a 6s2 electron pair exists. The inert pair effect is the tendency of this pair of electrons to resist oxidation due to a relativistic contraction of the 6s orbital. Wan et al. (2021) instead calculated a repulsive effect. The stability of gold and platinum anions in compounds such as caesium auride. The slightly reduced reactivity of francium compared with caesium. About 10% of the lanthanide contraction is attributed to the relativistic mass of high velocity electrons and the smaller Bohr radius that results.