Helium is the second-lightest element after hydrogen and the lowest-boiling, hardest-to-liquefy element of all, a colorless, odorless inert gas at room temperature with extraordinarily stable chemistry. Its largest high-end use is cryogenic cooling: liquid helium boils close to absolute zero and is the only practical coolant for superconducting magnets in hospital MRI scanners, particle accelerators, quantum-computing experiments and large research installations, without which these devices cannot operate. In deep-sea and saturation diving, helium-oxygen breathing mixtures replace nitrogen to prevent nitrogen narcosis and shorten decompression time, and are widely used on offshore oil and gas platforms and in salvage operations.
In welding, helium's inertness and high thermal conductivity make it the preferred shielding gas for arc welding aluminum alloys, stainless steel, copper and magnesium, as well as for the inert atmosphere used in semiconductor crystal growth and optical fiber drawing. Because helium atoms are tiny and chemically inert, it is also the gold-standard tracer gas for leak detection on air conditioners, refrigerators, gas cylinders, and welded joints of aircraft and spacecraft. In daily life, helium fills party balloons, blimps and inflatable toys: its density far below air gives buoyancy, and unlike hydrogen it does not burn. Helium is preferred precisely because it has an extremely low boiling point and extreme chemical inertness, remaining unreactive even at cryogenic extremes and around highly reactive metals.
Helium is a noble gas with a closed electron shell and extraordinarily stable chemistry; it forms almost no stable neutral compounds at all, so its industrial and civilian value comes almost entirely from the physical properties of the elemental gas itself—low density, low boiling point and chemical inertness. The only known helium species appear under extreme cryogenic, high-pressure or matrix-isolated laboratory conditions, such as hypothetical helium difluoride-type molecules and clathrates; they are unstable at ordinary temperature and pressure and have no practical use. What the market calls "helium compounds" generally refers to gas mixtures containing helium or products filled with helium, not chemical compounds in the strict sense.
