2He4.003
Helium
Noble gases · gas at STP · Block S
About
Helium is the second-lightest and second most abundant element in the universe. It's a noble gas, meaning it is chemically inert and forms almost no compounds. Famously used to fill balloons and airships due to its low density, it is also crucial in cryogenics for cooling superconducting magnets, such as in MRI scanners and particle accelerators. Most terrestrial helium is a product of radioactive decay.
Atomic Properties
Atomic Number2
Atomic Weight4.003 u
Neutrons (common)2
Electron Configuration1s²
Full Configuration1s²
Electrons per Shell2
Electron Affinity0 kJ/mol
Oxidation States0
Covalent Radius28 pm
Van der Waals Radius140 pm
Periodic Table Position
Group18
Period1
BlockS
Chemical SeriesNoble gases
Physical Properties
State at STPgas
AppearanceColorless gas
Density at STP0 g/cm³
Crystal Structurehcp
Magnetic Orderingdiamagnetic
Speed of Sound970 m/s
Thermal Properties
Melting Point0.95 K
Boiling Point4.22 K
Triple Point2.177 K
Thermal Conductivity0.151 W/(m·K)
Specific Heat5.193 J/(g·K)
Molar Heat20.786 J/(mol·K)
Heat of Fusion0.021 kJ/mol
Heat of Vaporization0.083 kJ/mol
Abundance
In Earth's Crust0.003 ppm
In Ocean0.007 ppm
In Solar System230,000 ppm
In Universe230,000 ppm
Discovery
Discovered byPierre Janssen & Norman Lockyer
Discovery Year1,868
Occurrenceprimordial
CAS Number7440-59-7
Electron Configuration
Energy
↿⇂
2s
2p
3s
3p
4s
3d
4p
5s
4d
5p
6s
4f
5d
6p
7s
5f
6d
7p
Isotopes of Helium
| Symbol ▲ | Mass (u) ▲ | Abundance ▲ | Half-life ▲ | Decay Mode ▲ |
|---|---|---|---|---|
| 3He | 3.016029 | 0.000137% | Stable | Stable |
| 4He | 4.002603 | 99.999863% | Stable | Stable |
| 5He | 5.012220 | — | 602 ys | Neutron Emission |
| 6He | 6.018889 | — | 806.92 ms | Beta Minus Decay |
Interactive Visualization
Explore the 3D Bohr model of Helium in our interactive viewer.
View 3D Model of Helium