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Helium is the chemical element with the symbol He and atomic number 2. It is a colourless, odorless and tasteless noble gas found in the second main group and first period of the periodic table of elements.
Helium is the second most abundant element in the Universe, accounting for about 23% of the mass of the observable Universe. However, it is very rare on Earth and is mainly extracted from natural gas (air) deposits. It is very light and has a lower density than air, which is why it is often used in balloons. Helium is also a very stable element and only reacts with other elements under very specific conditions.
Helium was discovered in 1868 by French astronomer Pierre Janssen while observing the solar eclipse. Janssen noticed that the sun's light was repeatedly absorbed by a specific element, which he identified as helium. A few years later, in 1895, the element was also discovered on Earth by British chemist Sir William Ramsay while studying the mineral krynite.
| 1s | 2s | 2p | 3s | 3p | 3d | 4s | 4p | 4d | 4f | 5s | 5p | 5d | 5f | 6s | 6p | 6d | 6f | 7s | 7p |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2 |
Abbreviated form: 1s2.
The following table lists the ionization energies IE (ionization potentials); the IE is the energy required in electron volts (eV) per atom to separate a given electron from an Helium atom.
| IE1 | IE2 |
|---|---|
| 24.587387 | 54.417760 |
An overview of the nuclides as well as the isotopic data and properties are listed on the following page: Helium isotopes.
Last update: 2022-12-21
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