Helium has the particularity of having been discovered in the Sun before being found on Earth. In 1868, during a solar eclipse, the French astronomer Pierre Janssen (1824-1907) observed an unknown yellow line in the solar spectrum. The same year, the British astronomer Norman Lockyer (1836-1920) identified this line and proposed that it belonged to a new element, which he named helium (from the Greek helios = sun). It was not until 1895 that the Swedish chemist Per Teodor Cleve (1840-1905) and independently William Ramsay (1852-1916) isolated helium on Earth from cleveite, a radioactive mineral.
Helium (symbol He, atomic number 2) is the first noble gas in the periodic table, consisting of two protons, two neutrons (for the most common isotope), and two electrons. The two main stable isotopes are helium-4 \(\,^{4}\mathrm{He}\) (≈ 99.999863%) and helium-3 \(\,^{3}\mathrm{He}\) (≈ 0.000137%).
At room temperature, helium is a monatomic gas (He), extremely light (density ≈ 0.1785 g/L), colorless, odorless, and completely chemically inert. The temperature at which the liquid and solid states can coexist (melting point): 0.95 K (−272.20 °C) at 2.5 MPa (helium does not solidify at atmospheric pressure). The temperature at which it transitions from liquid to gas (boiling point): 4.222 K (−268.928 °C) at atmospheric pressure.
| Isotope / Notation | Protons (Z) | Neutrons (N) | Atomic mass (u) | Natural abundance | Half-life / Stability | Decay / Remarks |
|---|---|---|---|---|---|---|
| Helium-3 — \(\,^{3}\mathrm{He}\,\) | 2 | 1 | 3.016029 u | ≈ 0.000137 % | Stable | Rare on Earth, more abundant in space; used in cryogenics and fusion research. |
| Helium-4 — \(\,^{4}\mathrm{He}\,\) | 2 | 2 | 4.002603 u | ≈ 99.999863 % | Stable | Major isotope; alpha nucleus emitted during radioactive decay; becomes superfluid below 2.17 K. |
| Helium-5 — \(\,^{5}\mathrm{He}\,\) | 2 | 3 | 5.012057 u | Unnatural | ≈ 7 × 10⁻²² s | Extremely unstable; rapidly decays into \(\,^{4}\mathrm{He}\) + neutron. |
| Helium-6 — \(\,^{6}\mathrm{He}\,\) | 2 | 4 | 6.018889 u | Unnatural | 0.807 s | Radioactive β\(^-\) decay to \(\,^{6}\mathrm{Li}\); artificially produced in laboratories. |
| Heavier isotopes — \(\,^{7}\mathrm{He},\,^{8}\mathrm{He},\,^{10}\mathrm{He}\) | 2 | 5 — 8 | — (resonances) | Unnatural | \(10^{-21}\) — 0.003 s | Very unstable states observed in nuclear physics; decay by neutron emission. |
N.B. :
Electron shells: How electrons organize around the nucleus.
Helium has 2 electrons distributed in a single electron shell. Its complete electronic configuration is: 1s², which can also be written as: K(2). Helium is the only stable element with a single complete electron shell.
K Shell (n=1): Contains 2 electrons in the 1s sub-shell. This single shell is complete and saturated, as the first shell can hold a maximum of 2 electrons. This configuration represents the most stable energy state possible for 2 electrons.
Helium has 2 electrons in its single shell, forming a saturated electronic configuration. This configuration explains its exceptional chemical properties:
Helium neither loses nor gains electrons under any conditions, which explains the total absence of oxidation states.
The complete valence shell gives helium absolute chemical inertness, hence its classification among the noble gases (or rare gases).
No helium chemical compound has ever been synthesized, even under extreme laboratory conditions. Helium is the most inert of all chemical elements, surpassing even neon.
The electronic configuration of helium, with its single complete shell of 2 electrons, makes it the most stable and inert element in the periodic table. This structure gives it exceptional characteristic properties: absolute chemical inertness (helium forms no compounds and reacts with no element), the highest ionization energy of all elements (it is practically impossible to remove an electron), the lowest boiling point of all elements (4.2 K or -269°C), and it is the only element that cannot solidify at atmospheric pressure, even at absolute zero.
Helium represents the most stable energy state for 2 electrons. Its configuration serves as a reference for describing elements of the second period of the periodic table. Many ions seek to achieve this stable [He] configuration by losing electrons (such as Li⁺, Be²⁺).
The importance of helium lies entirely in its exceptional physical properties: liquid helium is the ultimate cryogenic fluid, used to cool superconducting magnets in MRI machines, particle accelerators like the LHC, and in superconductivity research; helium gas is used to inflate balloons and airships due to its lightness (second lightest element after hydrogen) and non-flammability; it serves as a shielding gas in arc welding for reactive metals; helium is used in breathing mixtures for deep diving (heliox) because it is less soluble in blood than nitrogen, reducing the risks of narcosis and decompression sickness; it serves as a carrier gas in gas chromatography; helium is also used to detect leaks in vacuum systems due to its very small atomic size. Helium is the second most abundant element in the universe after hydrogen, produced by nuclear fusion in stars, but it is relatively rare on Earth because its lightness allows it to escape from Earth's atmosphere. Terrestrial helium reserves come from natural radioactive decay in the Earth's crust, trapped in certain natural gas deposits, making it a non-renewable and strategic resource.
Helium is the quintessential noble gas: its outer electron shell is complete, making it chemically inert. It forms virtually no stable chemical bonds under normal conditions. Even at very high pressure and low temperature, helium resists compound formation. This total inertness makes helium the most stable and non-reactive of all elements. However, helium can be trapped in complex molecular structures (inclusion compounds) or form ephemeral ionic molecules such as HeH⁺ (helium hydride ion), detected in the interstellar medium.
Helium accounts for about 24% of the baryonic mass of the universe, making it the second most abundant element after hydrogen. It was synthesized in large quantities during primordial nucleosynthesis, a few minutes after the Big Bang. In stars, helium is the main product of hydrogen fusion. When hydrogen is depleted in the stellar core, helium fusion begins at temperatures above 100 million kelvins, producing carbon and oxygen through the triple-alpha process.
The abundance of helium in the universe is a major piece of evidence for the Big Bang model. Precise measurements of the helium/hydrogen ratio help constrain cosmological parameters and test theories about the evolution of the early universe. Helium-3, although rare on Earth, is present in significant quantities in the solar wind and on the surface of the Moon, where it could one day be mined as fuel for nuclear fusion.
Liquid helium, particularly helium-4 below 2.17 K (lambda point), becomes superfluid: it flows without any viscosity and can climb the walls of containers. This spectacular behavior illustrates quantum effects on a macroscopic scale and has revolutionized our understanding of condensed matter.
N.B.:
Helium is a non-renewable resource on Earth. It is produced naturally by the radioactive decay of uranium and thorium in the Earth's crust, then trapped in certain natural gas deposits. Once released into the atmosphere, helium is so light that it escapes Earth's gravity and is lost to space. Current helium consumption far exceeds its natural production, raising concerns about its future availability for critical applications in medicine and scientific research.
