Helium (He) is a colorless, odorless, non-toxic gas with the atomic number two. It is the second lightest element, following hydrogen. As the first member of the noble gas group, Helium is characterized by profound chemical stability. This stability means it is extremely reluctant to react with other elements, a property that makes it unique. We explore the science behind this inertness and the rare exceptions where it interacts with other substances.
The Electron Structure Behind Inertness
The primary reason for Helium’s non-reactivity lies in its unique electron configuration, \(1s^2\). This means the atom has two electrons, which completely fill its innermost and only electron shell. This arrangement is known as a “duet configuration” and represents a state of exceptional chemical stability. Atoms typically participate in chemical reactions to achieve such a stable, full outer shell.
Because Helium already possesses this optimal structure, it has no energetic incentive to gain, lose, or share electrons with other atoms. Consequently, it exhibits an exceptionally high first ionization energy, requiring 24.57 eV of energy to remove a single electron. This tremendous energy barrier prevents the atom from easily becoming a positively charged ion. The atom’s small size and tightly bound electrons reinforce this stability, making it highly resistant to any chemical process under normal conditions.
The Absence of Stable Chemical Bonds
The question of what Helium reacts with can be answered with “nothing” under typical conditions of pressure and temperature. Helium’s inert nature is far greater than that of its heavier noble gas counterparts, such as Xenon, which have been observed to form several stable compounds. The fundamental issue is that any theoretical compound involving a neutral Helium atom would be less energetically stable than the two separate, non-bonded atoms.
Compounds Under Extreme Pressure
However, scientists have successfully forced Helium to interact under extreme, non-standard conditions. One notable example is the compound disodium helide (\(\text{Na}_2\text{He}\)), synthesized under ultra-high pressure in a laboratory setting. This compound is stable only when subjected to pressures in excess of 113 gigapascals, conditions that mimic the interiors of gas giant planets. In this high-pressure environment, the Helium atom is incorporated into an unusual crystal structure rather than forming a traditional covalent bond.
Transient and Theoretical Compounds
Another class of exception involves highly energetic, short-lived molecules called excimers, such as \(\text{He}_2\). These molecules can be temporarily formed when one of the Helium atoms is electronically excited, allowing a transient bond to exist. As soon as the excited electron drops back to its lower energy level, the molecule rapidly decays back into two separate, non-bonded atoms. Theoretical predictions suggest that compounds like \(\text{He}_3\text{F}_2\) could form stable covalent bonds at pressures in the tera-pascal range.
Practical Uses of Helium’s Stability
Helium’s refusal to react with other substances is a valuable asset in several industrial and scientific applications. Its chemical stability makes it an ideal shielding gas in arc welding, particularly for reactive metals like aluminum and stainless steel. A stream of Helium gas is directed at the weld area to displace atmospheric oxygen and nitrogen, preventing the metal from oxidizing and ensuring a clean, strong weld.
The gas is also used in specialized breathing mixtures for deep-sea divers, often combined with oxygen and nitrogen in a blend known as Trimix. Helium’s inertness means it is less soluble in the bloodstream and tissues than nitrogen, reducing the risk of nitrogen narcosis and mitigating the effects of high-pressure nervous syndrome.
Finally, Helium’s non-reactivity is paired with the lowest boiling point of any element (\(-268.9\) degrees Celsius), making it the premier choice for cryogenic cooling. Liquid Helium is used to cool the superconducting magnets found in medical devices like Magnetic Resonance Imaging (MRI) machines. It is also used to cool equipment in particle accelerators, ensuring that the components maintain the ultracold temperatures necessary for their operation.