Helium
[CAS# 7440-59-7]

Identification

• Classification: Inorganic chemical industry >> Industrial gases such as hydrogen, nitrogen and oxygen
• Name: Helium
• Synonyms: Atomic helium; Helium-4; o-Helium; p-Helium
• Molecular Formula: He
• Molecular Weight: 4.00
• CAS Registry Number: 7440-59-7
• EC Number: 231-168-5
• SMILES: [He]

Properties

• Density: 0.178 g/mL (Expl.)
• Melting point: -272.2 $degree$C (Expl.)
• Boiling point: -268.6$+/-$9.0 $degree$C 760 mmHg (Calc.)^*, -268.9 $degree$C (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS04 Warning
• Risk Statements: H280-H281
• Safety Statements: P282-P336+P317-P403-P410+P403

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Gases under pressure (compressed)	Press. Gas (Comp.)		H280
Gases under pressure (refrigerated liquid)	Press. Gas (Ref. Liq.)		H281

Discovery and Applications

Helium is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, and non-toxic noble gas, and it is the second lightest and second most abundant element in the observable universe after hydrogen. Helium was first discovered in 1868 during spectroscopic observations of the Sun, where a yellow spectral line was noted that did not match any known element, and it was later isolated on Earth in 1895.

Helium is chemically inert due to its complete valence electron shell, which makes it highly stable and non-reactive under standard conditions. Its low atomic mass and weak interatomic forces give it unique physical properties, including the lowest boiling point of any element at –268.93 °C, making it a valuable cryogenic agent. Helium remains a gas at temperatures and pressures where most other substances are liquids or solids, and it only solidifies under extremely low temperatures and high pressures.

One of the primary applications of helium is as a cryogenic coolant. Liquid helium is essential in cooling superconducting magnets, such as those used in magnetic resonance imaging (MRI) machines and particle accelerators, because it can maintain temperatures near absolute zero. Its low boiling point also allows for applications in scientific research requiring extremely low-temperature environments.

Helium is also widely used in gas applications due to its inertness. It serves as a protective gas in welding, as a carrier gas in gas chromatography, and as a filler in balloons and airships because it is lighter than air and non-flammable, unlike hydrogen. Additionally, helium is employed in leak detection systems, where its small atomic size allows it to penetrate tiny cracks and openings in vacuum systems or pressurized equipment.

In industrial and aerospace applications, helium is used as a pressurizing and purge gas due to its chemical stability and low density. It is also used in controlled atmospheres for growing silicon and germanium crystals, where an inert environment is crucial to prevent contamination. Furthermore, helium plays a role in respiratory medicine and diving mixtures, reducing the risk of nitrogen narcosis in deep-sea diving when combined with oxygen.

Overall, helium is a chemically inert, light, and extremely low-boiling noble gas with significant applications in cryogenics, industrial processes, scientific research, and safety-critical technologies. Its unique combination of chemical stability, low density, and extreme cold tolerance makes it indispensable across multiple scientific, medical, and industrial fields.

References

2025. Helium governs nitric oxide signaling to improve alfalfa salinity tolerance by reestablishing redox and ion homeostasis. Plant physiology and biochemistry : PPB.
DOI: 10.1016/j.plaphy.2025.110055

2025. Helium and argon cold plasma effects on the 4T1 cancer cells and a triple negative mouse model of breast cancer. Scientific Reports.
DOI: 10.1038/s41598-025-95065-z

2025. Positive Effects of Argon Inhalation After Traumatic Brain Injury in Rats. International Journal of Molecular Sciences.
DOI: 10.3390/ijms252312673