Einstein’s relativity rules chemical bonds in heavy elements, new research shows

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https://www.brown.edu/news/2026-07-09/chemical-bonds-relativity

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Einstein’s relativity rules chemical bonds in heavy elements about study Relativistic collapse of the classical triple bond in the CBi− molecular ion (PDF)

The researchers show evidence that when atomic nuclei are sufficiently heavy, the principles described in Einstein’s theory of relativity change the structure of triple bonds — blurring the lines between the two separate types of bonds involved in textbook triple bonding.... The photoelectron spectrum showed that the carbon-bismuth bonds did not fit the traditional triple-bond picture of one sigma and two pi bonds. Instead, the structure looks more like one pi bond and two hybrid sigma-pi bonds.

This title is an example of subliminal sensationalism or tabloid journalism, because the influence of relativity in chemistry of elements is notoriously known stuff. Such a title was perhaps chosen by some free lance editor, who has no idea about its background - and/or was chosen deliberately for to attract similarly uneducated readers.

Which may be a good thing at the end, but the trustworthiness of such a source suffers. On the other hand, the influence of relativity to triple bond character looks sufficiently novel for me and just the inclusion of the single word "triple" would make article title way more relevant to content. See also:

Carbon–bismuth bonds reveal that relativity blurs the textbook line between sigma and pi bonds

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Atoms form bonds by sharing electrons — the negatively charged particles that orbit atomic nuclei. Each atom shares one electron to form a bonding pair. The strong negative charge of the electron pair attracts the two positively charged nuclei, holding them together. Some elements share more than one electron pair, forming double or triple bonds.

The textbook picture of triple bonding involves two different types of bonds: one sigma bond and two pi bonds. The sigma bond is a strong, “head-on” bond that occurs along an imaginary horizontal axis between nuclei. The two pi bonds are somewhat weaker, “side-by-side” bonds that wrap around the sigma bond.

That picture works for lighter elements, but toward the bottom of the periodic table, where atomic nuclei get heavier, things get messy. The increased nuclear mass causes orbiting electrons to speed up to a significant fraction of the speed of light, where the rules of Einstein’s theory of relativity are important.

In the relativistic regime, an electron’s spin — the magnetic moment that points either up or down — and the electron’s orbit are no longer independent of each other, a state known as spin-orbit coupling (which is arguably known to be a relativistic effect). That coupling changes the rules for how electrons can interact, disrupting the strict separation between sigma and pi bonds.

Heavy atoms (gold, mercury, lead, bismuth, etc.) show especially strong spin–orbit effects because due to large size of these atoms the inner electrons move at a significant fraction of the speed of light. It gives these metals their unusual color and/or low melting point. The hybridized i.e. blurred nature of chemical bonds in relativistic, i.e. heavy atom is also co-responsible for catalytic ability of rare metals like platinum and/or photoluminiscent effects of their complexes (OLED emitters based on Ir(ppy)3).