Quick Facts
| Property | Value |
|---|---|
| Symbol | He |
| Atomic Number | 2 |
| Atomic Mass | 4.0026 u |
| Category | Noble Gas |
| Period | 1 |
| Group | 18 |
| Block | s |
| Electron Configuration | 1s² |
Physical Properties
| Property | Value |
|---|---|
| State at 20°C | Gas |
| Density | 0.0001786 g/cm³ |
| Melting Point | -272.20°C (0.95 K) at 25 atm |
| Boiling Point | -268.93°C (4.22 K) |
| Appearance | Colorless, odorless gas |
Atomic Properties
| Property | Value |
|---|---|
| Electron Configuration | 1s² |
| Electronegativity | N/A (no stable compounds) |
| First Ionization Energy | 2372.3 kJ/mol |
| Atomic Radius | 31 pm |
| Covalent Radius | 28 pm |
| Van der Waals Radius | 140 pm |
History and Discovery
Discovered by: Pierre Janssen and Joseph Norman Lockyer Year of Discovery: 1868 Location: France and England
Etymology
The name "helium" derives from the Greek word "helios" meaning "sun," as it was first detected in the solar spectrum during a solar eclipse.
Discovery Story
Helium was discovered spectroscopically in the Sun before it was found on Earth. On August 18, 1868, French astronomer Pierre Janssen observed a yellow spectral line during a solar eclipse in India. Independently, English astronomer Joseph Norman Lockyer observed the same line and concluded it represented an unknown element. In 1895, Scottish chemist William Ramsay first isolated helium on Earth from the uranium mineral cleveite.
Isotopes
| Isotope | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|
| ³He | 0.0002% | Stable | - |
| ⁴He | 99.9998% | Stable | - |
Occurrence
Natural Abundance
Helium is the second most abundant element in the observable universe after hydrogen, making up about 24% of total elemental mass. However, on Earth it is relatively rare, comprising only about 5.2 ppm of the atmosphere. Significant deposits exist in natural gas fields, particularly in the United States, Qatar, Algeria, and Russia.
Extraction and Production
- Natural Gas Processing: Most helium is extracted from natural gas fields where it occurs as a byproduct of radioactive decay of uranium and thorium in the Earth's crust
- Air Separation: Small amounts can be obtained from liquefied air, though this is not economical
- Locations: The US Federal Helium Reserve in Texas was the world's largest supplier for decades
Applications and Uses
Industrial Applications
- Inert shielding gas for arc welding (especially aluminum and stainless steel)
- Leak detection in high-vacuum systems and containers
- Pressurizing and purging systems
- Growing silicon and germanium crystals for semiconductors
Scientific Applications
- Cryogenics (cooling superconducting magnets in MRI machines)
- Particle physics experiments
- Low-temperature research (superfluidity studies)
- Gas chromatography carrier gas
Technological Applications
- Cooling superconductors in MRI and NMR machines
- Fiber optic cable manufacturing
- Rocket propulsion (pressurizing fuel tanks)
Other Uses
- Inflating balloons and airships (safer alternative to hydrogen)
- Deep-sea diving breathing mixtures (heliox)
- Voice alteration (temporarily raises voice pitch)
Biological Role
Helium has no known biological role and is not toxic. It is chemically inert and does not participate in any biochemical processes.
In the Human Body
Helium is not metabolized or used by the body. When inhaled, it is simply exhaled unchanged.
Medical Uses
Used in heliox breathing mixtures for patients with severe respiratory conditions, as it reduces breathing resistance due to its low density.
Safety and Hazards
Toxicity
Helium is non-toxic but can act as an asphyxiant by displacing oxygen in enclosed spaces.
Handling Precautions
- High-pressure gas cylinders require proper handling
- Can cause asphyxiation in confined spaces without adequate ventilation
- Inhalation of helium for voice effects can be dangerous if oxygen is displaced
- Rapid decompression can cause frostbite
Environmental Impact
Helium is environmentally inert and does not contribute to air pollution or climate change. However, helium is a non-renewable resource that escapes into space once released.
Interesting Facts
- Helium is the only element that cannot be solidified by cooling at standard pressure—it requires about 25 atmospheres of pressure
- Below 2.17 K, helium becomes a superfluid with zero viscosity and can flow through microscopic pores
- Helium is so light it can escape Earth's atmosphere into space
- The "helium shortage" is a real concern as global reserves are limited
- Helium has the lowest boiling point of any element