Helium is the second lightest element and a noble gas, a group of elements resistant to chemical reactions. Chemical bonding involves the sharing or transfer of electrons between atoms to achieve a stable state. Under normal Earth conditions, helium does not participate in this process, leading to the belief that it could not form compounds. However, scientific exploration reveals that while the answer is typically “no,” there are fascinating and extreme exceptions.
Helium’s Unique Atomic Stability
Helium’s chemical inertness stems from its unique electron configuration, achieving maximum stability with minimal electrons. A helium atom has two electrons, which completely fill its single electron shell. This arrangement is referred to as the duet rule, a specialized version of the stability goal most other elements achieve through eight electrons.
The filled shell means the atom is energetically satisfied and has no drive to gain, lose, or share electrons. Disrupting this configuration requires a massive input of energy. Helium possesses the highest first ionization energy of any element, meaning it takes more energy to remove one of its electrons than any other atom.
This exceptional stability ensures that under ambient conditions, helium atoms simply bounce off one another without reacting, as weak attractive forces are overpowered by natural repulsive forces between electron shells.
Extreme Environments and Exotic Bonding
To overcome helium’s stability, scientists must introduce non-traditional conditions that force the atom to react. One method involves providing a tremendous energy boost, such as through ionization. Removing an electron creates a positively charged helium ion (\(\text{He}^+\)), which is highly unstable and has an electronic configuration similar to a reactive hydrogen atom.
Another method involves subjecting helium to extreme physical conditions, particularly immense pressure, such as those found deep inside planets. High pressure forces atoms into much closer proximity, overcoming natural repulsion and altering electron orbital overlap. This compression fundamentally alters the electronic structure and reactivity of an element, allowing for bonding mechanisms impossible at normal atmospheric pressure.
These extreme conditions provide the necessary energy or structural changes to push helium into a bonded state. Forcing a reaction under high pressure can lead to the formation of crystalline solids where helium atoms are mechanically incorporated into a lattice structure. These conditions, characterized by high pressure and high temperature, are thought to exist in the cores of certain planets and stars.
Confirmed and Predicted Helium Compounds
The most famous and well-established example of helium bonding is the helium hydride ion, \(\text{HeH}^+\). This positively charged ion consists of a helium atom bonded to a hydrogen atom, and it holds the distinction of being the first molecular bond that formed in the Universe after the Big Bang. The ion was first created in a laboratory setting in 1925, but its existence in space was not confirmed until 2019, detected in a planetary nebula.
Under the extreme pressure conditions discussed, several solid compounds involving helium have been created or predicted. One remarkable example is disodium helide, \(\text{Na}_2\text{He}\), a crystalline compound synthesized by compressing sodium and helium to pressures above 113 gigapascals. This material is not a traditional compound with covalent or ionic bonds between helium and sodium atoms; instead, it is an electride, where the sodium lattice holds isolated pairs of electrons in spaces within the crystal structure. The helium atoms themselves are physically trapped and are required to stabilize the unique structure.
Other predicted or observed high-pressure solid phases include silicates and arsenolite inclusion compounds, where helium atoms are physically incorporated into the cages of a crystal lattice. Research has also shown that helium can react with molten iron under high pressure and temperature to form stable compounds, providing support for the hypothesis that helium isotopes are trapped in the Earth’s core.