Most people learn in school that chemical bonds have a specific structure. A triple bond usually has one strong sigma bond and two weaker pi bonds. However, in the heaviest elements, electrons move so quickly that these rules do not apply.
For the first time, scientists from Brown University and Washington State University have found experimental evidence that forces from Albert Einstein's theory of relativity change how chemical bonds work. This discovery, published in the journal Science, confirms an idea scientists have had since the 1970s.
The research focused on bismuth, a heavy element. Because bismuth has a large nucleus, it pulls strongly on its inner electrons. These electrons orbit at speeds close to the speed of light. At these speeds, Einstein's special relativity becomes important.
"This idea that relativity is important in heavy elements has been around since the 1970s," said Lai-Sheng Wang, a professor at Brown University. "But we show direct evidence that what we learned in high school about chemical bonding isn't true in heavy elements."
The team studied a molecule made of carbon and bismuth. They froze the samples to very low temperatures to get accurate measurements. Using a technique called photoelectron spectroscopy, they fired lasers at the molecules to remove electrons. This helped them measure how strongly the atoms were connected.
They discovered that the fast-moving electrons caused a phenomenon called spin-orbit coupling. This effect blurred the lines between different types of bonds. Instead of the usual sigma and pi bonds, the carbon-bismuth bond had a mix of characteristics.
"The boundary between a sigma bond and a pi bond is now sort of smeared," Wang explained. "We still have three bonds, but we don't really strictly have a sigma or a pi anymore."
Kirk Peterson, a scientist at Washington State University, used computer calculations to explain the results. His calculations matched the experimental data and showed that relativity changes the molecule's symmetry.
This discovery was unexpected. The scientists were originally studying something else when they found this unusual data. It shows that the rules of chemistry may need to be updated for heavy elements, and this knowledge could help in developing new technologies, like safer solar panels.