Helium (He)
Atomic #2 Group #18

Second lightest and second most abundant element.


Helium discharge tube
Helium discharge tube (Photo by Alchemist-hp, CC-BY-NC-ND-3.0)

Word

Pronunciation

/ˈhiːliəm/ ​(HEE-lee-əm)

Appearance

colorless gas, exhibiting a gray, cloudy glow (or reddish-orange if an especially high voltage is used) when placed in an electric field

Chemical properties

Phase at STP

Gas

Melting point

0.95 K ​(−272.20 °C, ​−457.96 °F) (at 2.5 MPa)

Boiling point

4.222 K ​(−268.928 °C, ​−452.070 °F)

Density

0.1786 g/L (STP), 0.145 g/cm3 (liquid, melting point), 0.125 g/cm3 (liquid, boiling point)

Atomic Weight

4.002 amu

History and etymology

Etymology

Named after Helios, Greek Titan of the Sun

Discovered

Pierre Janssen, Norman Lockyer (1868)

First isolation

William Ramsay, Per Teodor Cleve, Abraham Langlet (1895)

Isotopes

3He (stable), 4He (stable)

Quantum

Electron configuration

1s2

Orbital diagram

1s[↑↓]

Quantum numbers of the last electron

n=1, l=0, Ml=0, Ms=-0.5