A group of scientists from the US and China has determined that under extreme pressure, it might be possible for stable covalent bonds between helium and fluorine to occur, contradicting the notion that helium is a chemically inactive element.
Noble gases are named for their natural hesitance to react. Their complete electron structures result in very high ionisation energies, leading to a reluctance to create compounds. Nevertheless, there have been various instances of heavier noble gas compounds forming in non-standard conditions, particularly a series of xenon fluorides. In contrast, helium equivalents are significantly less common, with such compounds typically being unstable or only briefly existing as transition states.
Currently, through an algorithmic search that alters the chemical makeup and pressure, researchers have discovered a compound where helium is actively involved in chemical bonding. At pressures reaching the tera-pascal range – approximately 10 times the pressure at the Earth’s core – a stable compound He₃F₂ is formed. This molecule is made up of HeF₂ chains and interstitial helium atoms. Within each chain, helium forms polar covalent bonds with three fluorine atoms, with helium contributing electron charge. Molecular orbital computations of each HeF₃ cluster indicate that extreme pressure enables bonding between the helium 1s and fluorine 2p orbitals.
The discovery of helium–fluorine bonding further disputes the belief that noble gases are nonreactive. The researchers suggest that helium-containing interiors of giant planets might possess similar compounds. The extreme pressures required to create these materials might be replicable in laboratory settings, although only at a limited number of specialized research facilities.