Rules Are Only Suggestions in Heavy Elements

The arrangement of electrons in an exotic human-made element shows that certain properties of heavy elements cannot be predicted using lighter ones.

heavy elements
The application of Hund’s First Rule to berkelium(IV) leads to the expectation of an even distribution of parallel electrons (purple and peach) in each of the orbital subshells. This rule assumes that electron-electron repulsion is much larger than spin-orbit coupling. This rule does not hold true in heavy elements and a large deviation from the rule is observed in berkelium(IV) iodate. Image courtesy of Journal of the American Chemical Society

The Science

The rules that are taught for predicting the arrangement of electrons in light elements do not apply well to heavy elements. Why? Because relativistic effects are much larger in an element such as berkelium than in carbon or iron. Relativistic effects follow from Einstein’s theory of relativity; it states the faster something with mass moves, the heavier it gets. Berkelium’s nucleus causes its electrons to move at significant fractions of the speed of light due to its massive positive charge. The speeding electrons gain mass, and the rules that typically apply to electron behavior break. This means that the chemical and physical properties of heavy elements cannot be predicted using lighter ones even if they have the same number of outer electrons (for example, gadolinium(III) vs curium(III) vs berkelium(IV)).

The Impact

The safe use of nuclear power and environmental remediation of the Cold War legacy waste requires a deep understanding of the elements most involved in these processes, such as uranium and plutonium. Understanding how relativity changes the chemical properties of these elements may lead to improved recycling of used nuclear fuel. It could also lead to acceleration of the clean-up of nuclear sites in the United States.


The synthesis and characterization of two berkelium compounds creates rare opportunities to probe the electronic structure of some of the last elements that can be prepared on a macroscopic scale. Examination of the structure and properties of berkelium(III) and berkelium(IV) iodate, using both experiment and theory show, several unanticipated results that include shorter than expected berkelium-oxygen bonds and reduced magnetic moments. Theoretical examination of the ground states of these ions shows that they deviate from the spin-orbit coupling normally seen, and that the ground states are mixtures of electronic states instead of being just one. This admixture is created by large spin-orbit coupling that is consequence of the enhanced relativistic effects in heavy elements.

Source : U.S. Department of Energy