Helium (He)
Atomic #2 Group #18
Second lightest and second most abundant element.

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