Lithium-11

Every so often, Molecule of the Week informs you about substances that are not actual molecules. Here, we bring you the strange case of Lithium-11 (¹¹Li)¹. How is it possible for an atomic nucleus that contains only three protons to hold eight neutrons?

Back in 1966, researchers at the Lawrence Radiation Laboratory at the University of California, Berkeley, bombarded uranium nuclei with high-energy protons and detected what they called particle-stable nuclei of lithium-11, boron-14, and boron-15. Then, in the early 1990s, Russian physicists, led by M. V. Zhukov, described the ¹¹Li nucleus as having a “halo” of two loosely bound valence neutrons surrounding a cluster of protons and the other six neutrons. They named the structure “Borromean” after the three interlinked Borromean rings, which come apart when only one ring is broken.

Even though the ¹¹Li nucleus has a measure of stability, the isotope’s half-life is only ≈9 ms. The nucleus is also distinguished by its exceptionally large cross-sectional area—3.16 fm²—about the same size as the lead nucleus that contains almost 200 more nuclear particles.

Work on ¹¹Li and other unexpected nuclei continues. A soon-to-be-launched particle accelerator at Michigan State University (East Lansing), called the Facility for Rare Isotope Beams, is designed for deeper investigations into nuclei with extraordinarily large numbers of neutrons.

1. CAS Reg. No. 14993-29-4.