Helium

Helium is the chemical element with atomic number 2, a colorless, odorless noble gas. It has the lowest boiling point of any element and remains liquid at ordinary pressure only near absolute zero.

Atomic number: The number of protons in an atom’s nucleus, which determines its chemical element. Helium’s atomic number is two because every helium nucleus contains two protons.
Pauli exclusion principle: A quantum rule forbidding identical fermions from occupying the same quantum state. It helps explain the distinct low-temperature behavior of helium-3 atoms.
Cryogenics: The science and technology of producing and using very low temperatures. Liquid helium reaches temperatures needed for experiments and equipment beyond most refrigerants.
Jules Janssen: A French astronomer who observed a yellow spectral line in sunlight during the 1868 solar eclipse. His observation helped reveal helium in the Sun before its terrestrial discovery.
Helium shortage: A recurring mismatch between helium supply and demand that disrupts access to the gas. Limited sources and specialized uses make supply disruptions consequential.
Noble gas: An element in group 18 of the periodic table, generally characterized by low chemical reactivity. Helium belongs to this group, though its electron shell differs from those of heavier members.
Van der Waals force: A weak attraction arising from interactions between fluctuating or permanent electric dipoles. Weak attractions between helium atoms help account for its exceptionally low boiling point.
Magnetic resonance imaging: A medical imaging technique that uses strong magnetic fields and radio waves to produce internal images. Liquid helium cools the superconducting magnets in many MRI systems.
Norman Lockyer: An English astronomer who identified a new solar spectral line in 1868 and named its source helium. He interpreted the line as evidence for an element not then known on Earth.
Helium reserve: A natural or stored accumulation of helium that can be extracted or supplied for use. Geological reserves are the main source of helium for most commercial applications.

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