Helonium is another name for the helium hydride ion, HeH⁺, a positively charged molecular ion made from helium and hydrogen. It is not a chemical element, despite its name sounding similar to helium or other elements ending in “-onium.”
The term is mainly encountered in chemistry, molecular physics, spectroscopy, and astronomy. HeH⁺ is particularly important because it is considered a key molecule in the chemistry of the early universe, and astronomers finally confirmed its presence in space in 2019.
What Is Helonium?
Helonium refers to the helium hydride cation, whose chemical formula is HeH⁺. The official ChEBI database identifies the species as hydridohelium(1+) and lists HeH⁺ and helium hydride ion among its names.
The formula is straightforward:
- He = one helium atom
- H = hydrogen, effectively a proton in this molecular ion
- ⁺ = an overall positive electrical charge
This makes HeH⁺ a molecular cation, meaning it is a molecule-like species carrying a positive charge. It is also a heteronuclear diatomic ion because it contains two atoms from different elements.
Helonium should therefore not be confused with helium itself. Helium is an element, while helonium is a charged chemical species made from helium and hydrogen.
Is Helonium a Chemical Element?
No. Helonium is not an element and does not have an atomic number or a position on the periodic table.
Helium is the chemical element with the symbol He and atomic number 2. Helonium instead describes HeH⁺, a molecular ion containing helium and hydrogen. The distinction is important because the word “helonium” can sound like the name of an undiscovered element.
The recognized scientific identity of the species is supported by the ChEBI database, which gives its systematic name as hydridohelium(1+), formula HHe, and net charge +1.
Helonium Formula and Basic Properties
The chemical formula of helonium is HeH⁺. Its average mass is about 5.011 atomic mass units, according to ChEBI.
| Property | Helonium |
|---|---|
| Common name | Helium hydride ion |
| Other name | Helonium |
| Formula | HeH⁺ |
| Systematic name | Hydridohelium(1+) |
| Type | Molecular cation |
| Charge | +1 |
| Elements present | Helium and hydrogen |
| Chemical element? | No |
| ChEBI ID | CHEBI:33688 |
The positive charge gives HeH⁺ chemical behavior very different from ordinary neutral helium. Helium is famously chemically unreactive under normal conditions, whereas HeH⁺ is a highly reactive ionic species.
How Does Helonium Form?
A simplified description of its formation is the interaction of a helium atom with a proton:
He + H⁺ → HeH⁺
The actual chemistry can involve several reaction pathways depending on the environment. In the early universe, HeH⁺ could form when neutral helium interacted with protons during the period when the first atoms and molecules were developing. Research published in Nature describes HeH⁺ formation through radiative association between neutral helium and protons.
Because helium and hydrogen were the two dominant light elements produced in the early universe, their interactions are important for understanding the first stages of molecular chemistry.
Why Is Helonium Important in the Early Universe?
HeH⁺ has an unusual place in cosmic history. After the universe cooled sufficiently for atoms to form, helium was able to interact with protons and produce HeH⁺. This makes the ion an important part of models describing the earliest molecular chemistry.
The 2019 Nature detection paper explains that HeH⁺ formed in the early universe and that its destruction helped create a pathway toward molecular hydrogen, H₂. Molecular hydrogen later became an important component of the chemistry and cooling processes associated with the formation of astronomical structures.
This does not mean HeH⁺ was the only chemical species involved. Rather, it is an important link in the network of reactions that researchers use to model the transition from an almost entirely atomic universe to one containing increasingly complex molecules.
When Was Helonium Discovered?
The helium hydride ion was first experimentally observed in 1925. Researchers Thorfinn Hogness and E. G. Lunn detected an ion corresponding to helium hydride while studying ionized mixtures containing helium and hydrogen. Historical literature identifies their 1925 observations as the first experimental evidence for HeH⁺.
For decades, scientists also expected that HeH⁺ might exist naturally in astronomical environments. However, detecting it was difficult because its characteristic radiation occurs at wavelengths that are challenging to observe from the ground.
When Was Helonium Detected in Space?
Astronomers announced the first unambiguous astronomical detection of HeH⁺ in 2019.
The observation was made in the planetary nebula NGC 7027 using the SOFIA airborne observatory. Researchers detected the ion’s rotational transition at a wavelength of approximately 149.1 micrometres.
The discovery was significant because it provided direct observational confirmation that a molecule central to models of early cosmic chemistry can exist in an astronomical environment today.
The observation also helped scientists investigate the chemical reactions responsible for forming and destroying HeH⁺ in astrophysical environments.
Where Is Helonium Found?
Helonium is not a substance that normally accumulates in everyday environments. Instead, it is associated with specialized laboratory experiments and particular astrophysical conditions.
It can occur in:
- Laboratory plasmas and ion sources, where helium and hydrogen are ionized.
- Planetary nebulae, including NGC 7027.
- Astrophysical models of the early universe, where HeH⁺ is an important intermediate in primordial chemistry.
- Spectroscopy experiments, where researchers measure its molecular transitions and structure.
Its presence in space is especially useful because molecules emit and absorb radiation at characteristic frequencies. Those signals allow astronomers to identify molecular species even when the molecules themselves are unimaginably far away.
Why Is HeH⁺ Difficult to Study?
Helonium is extremely simple chemically, but its simplicity does not make it easy to study.
HeH⁺ is highly reactive. Under ordinary laboratory conditions, it readily reacts with other species, making it difficult to store as an ordinary bulk material. Researchers therefore study it using specialized ion traps, discharge systems, spectroscopic instruments, and theoretical calculations.
Its molecular transitions are also important for astronomy. Accurate measurements of these transitions allow scientists to distinguish HeH⁺ signals from other radiation sources.
What Makes Helonium Scientifically Interesting?
One reason HeH⁺ receives disproportionate attention is that it is an exceptionally simple molecular system. It contains only two nuclei and two electrons, yet accurately describing its structure requires sophisticated quantum-mechanical calculations.
This makes it useful as a benchmark for molecular theory. Scientists can compare extremely precise calculations with laboratory measurements and use the differences to test how well their physical models describe molecular systems.
Research published in Physical Review A in March 2026 calculated relativistic and quantum-electrodynamic corrections for HeH⁺. The authors reported theoretical transition energies with uncertainties low enough to support comparisons with high-precision spectroscopy and described HeH⁺ as an important benchmark for molecular structure theory, astrophysical modeling, and tests of fundamental physics.
Recent Helonium Research in 2026
Research on HeH⁺ remains active in 2026 rather than being limited to its historical role.
In January 2026, researchers published new measurements of the fundamental J = 1 ← 0 rotational transition of HHe⁺ at approximately 2.010 THz. The experiment used a cryogenic ion-trap system and a high-power frequency-multiplied terahertz source to refine measurements of this important transition.
Another 2026 study investigated the photoelectric effect of helonium under an extreme-ultraviolet laser pulse. The work used quantum-mechanical calculations to examine photoelectron distributions and electronic behavior in HeH⁺.
There has also been recent work on larger helium-hydrogen ion clusters. A 2026 RSC Advances study examined proton-driven interactions and structural organization in HeₙH⁺ clusters, extending research beyond the simplest HeH⁺ system.
These studies show that helonium remains relevant not only to astronomy but also to precision spectroscopy, quantum chemistry, molecular physics, and computational science.
Helonium vs. Helium
The names are similar, but the substances are fundamentally different.
| Feature | Helium | Helonium |
|---|---|---|
| Symbol/formula | He | HeH⁺ |
| Type | Chemical element | Molecular ion |
| Contains hydrogen? | No | Yes |
| Electrical charge | Neutral | +1 |
| Atomic number | 2 | Not applicable |
| Periodic-table element | Yes | No |
| Typical context | Gas, cryogenics, industry, science | Chemistry, spectroscopy, astrophysics |
Helium is a noble gas and is chemically stable under ordinary conditions. Helonium is a positively charged molecular ion and is much more reactive.
Helonium vs. Helionium
Another possible source of confusion is helionium, which is not the same term as helonium.
Helonium refers to HeH⁺, the helium hydride ion. If you encounter “helionium” in a physics or speculative-science context, it should not automatically be treated as a synonym for HeH⁺.
For scientific searches, the formula HeH⁺ is the safest way to identify the intended species.
Is Helonium Used in Everyday Life?
No. Helonium is not a household chemical, commercial gas, fuel, medicine, or commonly handled industrial material.
Its importance is primarily scientific. Researchers study it because its simple structure makes it useful for investigating chemical bonding, molecular spectroscopy, quantum mechanics, astrophysical chemistry, and the history of molecular formation in the universe.
Claims that describe helonium as an ordinary commercial material or a newly discovered chemical element are not consistent with the established scientific definition.
Does Helonium Have Practical Uses?
Helonium does not have widespread consumer or industrial applications. Its main value is as a research system.
Scientists use HeH⁺ to:
- Test high-precision molecular calculations.
- Study molecular bonding and electronic structure.
- Improve spectroscopic measurements.
- Model chemical reactions in astrophysical environments.
- Investigate the chemistry of the early universe.
- Develop and test theoretical approaches to quantum molecular systems.
The 2026 spectroscopy research is particularly relevant because increasingly accurate HeH⁺ measurements can provide better reference data for both laboratory physics and astronomical observations.
What Does “Helonium” Mean in Simple Terms?
In simple terms, helonium means HeH⁺, the positively charged helium hydride ion.
It is made from helium and hydrogen, but it is not a new element. Its importance comes from the unusual chemistry of helium, its role in models of early cosmic chemistry, its confirmed presence in space, and its value as a very simple system for testing advanced physical theories.
Frequently Asked Questions About Helonium
What is helonium?
Helonium is another name for the helium hydride ion, HeH⁺. It is a positively charged molecular ion containing helium and hydrogen.
Is helonium an element?
No. Helonium is not an element and has no atomic number on the periodic table. It is a molecular ion made from the elements helium and hydrogen.
What is the formula for helonium?
The formula is HeH⁺. The plus sign indicates that the molecular ion has an overall positive charge.
When was helonium discovered?
HeH⁺ was first observed experimentally in 1925 by Thorfinn Hogness and E. G. Lunn during experiments involving ionized helium and hydrogen.
Was helonium found in space?
Yes. Astronomers reported the first unambiguous detection of HeH⁺ in space in 2019, observing it in the planetary nebula NGC 7027 using SOFIA.
Why is helonium important?
HeH⁺ is important because it provides insight into molecular chemistry in the early universe and serves as a simple benchmark for precision molecular physics. Current research continues to refine its spectroscopic properties and theoretical description.
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