Periodic Table


Atomic number: 2
Atomic weight: 4.002602
Symbol: He
Group number: 18
Electronic configuration: 1s2


Helium is one of the so-called noble gases. Helium gas is unreactive, colourless, and odourless. Helium is available in pressurised tanks.

Elemental helium is a colourless odourless monoatomic gas. Helium is the second most abundant element in the universe after hydrogen. a particles are doubly ionised helium atoms, He2+.

Helium is used in lighter than air balloons and while heavier than hydrogen, is far safer since helium does not burn. Helium is also good for making your voice squeaky after taking a gulp from a balloon.

General information

Discoveror: Sir William Ramsay and independently by N. A. Langley and P. T. Cleve
Date discovered: 1895
Discovered at: London, England and Uppsala, Sweden
Meaning of name: From the Greek word "helios" meaning "sun"

Physical data

Standard state: gas at 298 K
Colour: colourless
Density of solid at ambient temperature/kg m-3: no data
Molar volume/cm3: 21.0

Radii /pm

Atomic: 31
Covalent (single bond):
Pauling radius for the ion [He]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius30.1no datano datano data


Both values are quoted on the Pauling scale.

Pauling: no data
Allred Rochow: 5.50

Crystal Structure

structure: ccp (cubic close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 4.22
boiling point: 4.22

Enthalpies /kJ mol-1

fusion: 0.018
vaporization: 0.083

single bond enthalpies:
He-F He-Cl He-Br He-I He-He
no data no data no data no data 3.8

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
3He3.01602931 (4)0.000137 (3)
4He4.00260324 (5)99.999863 (3)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/He.html