Helium (He)

Helium was found in the Sun before it was found on Earth: Janssen and Lockyer spotted an unexplained yellow line in the 1868 solar eclipse spectrum, and the element was named for Helios twenty-seven years before anyone isolated it from a terrestrial mineral. It is the only substance that will not freeze at any temperature under its own vapour pressure — you need about 25 atmospheres to make solid helium — and below 2.17 K it becomes a superfluid that climbs the walls of its own container. Essentially all of the helium on Earth is alpha particles from uranium and thorium decay, trapped in natural gas fields over hundreds of millions of years.

Properties

Atomic number

2

Symbol

He

Standard atomic weight

4.0026 u

Category

Noble gas

Group

18

Period

1

Block

s-block

Electron configuration

1s²

Electrons per shell

2

Melting point

0.95 K — -272.2 °C — -458.0 °F

Boiling point

4.22 K — -268.9 °C — -452.1 °F

Density

0.0001785 g/cm³

Oxidation states

0

Discovered

1868 by Pierre Janssen and Norman Lockyer

Electron configuration

Full

1s²

Noble gas shorthand

1s²

Shells

K: 2

What it is used for

• Cooling the superconducting magnets in MRI scanners
• Shielding gas for arc welding and semiconductor manufacture
• Leak detection in vacuum systems and air-conditioning plant
• Lifting gas for weather balloons and airships
• Deep-diving breathing mixes, where it replaces nitrogen

The melting point given is under about 2.5 MPa of applied pressure; helium does not solidify at all at ordinary pressure. Density is the gas at 0 °C and 101.325 kPa.

Neighbours in the table

Group 18

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Helium sits in group 18 with neon, argon, krypton, xenon, radon, oganesson.