Helium is a nonmetal. Under every condition you will ever encounter on Earth, helium behaves as an inert, colorless, odorless gas with no tendency to conduct electricity, form metallic bonds, or do anything chemically interesting at all. It sits in the upper-right corner of the periodic table among the noble gases, and its two tightly held electrons make it the least reactive element known. Yet the story has a fascinating wrinkle: squeeze helium hard enough and the distinction between nonmetal and metal starts to blur.
What Makes Helium a Nonmetal
The classification of elements into metals, nonmetals, and metalloids is based on a cluster of physical and chemical properties. Metals conduct electricity and heat well, they are malleable, they tend to lose electrons, and in solid form they have that characteristic luster. Nonmetals are the opposite in most respects: poor conductors, brittle as solids, and they tend to gain or share electrons rather than give them up. Metalloids sit on the border and display a mix of both sets of traits.
Helium checks every nonmetal box decisively. It has the highest ionization energy of any element, meaning its electrons are extraordinarily difficult to strip away. It has no metallic bonding, no electrical conductivity, and no tendency to form cations. Its boiling point is the lowest of any element, just 4.2 degrees above absolute zero. Even compared to other nonmetals, helium is an extreme case: it is so chemically inert that it forms no stable compounds under ordinary conditions. There is no ambiguity in the classification at standard temperature and pressure.
The Periodic Table Debate
Helium’s position on the periodic table is less settled than you might expect. The standard table places helium at the top of Group 18, above neon and the other noble gases. This makes sense chemically: helium is inert, neon is inert, they share a complete outer electron shell and similar disinterest in bonding. But helium has two electrons in an s-orbital, not a filled p-orbital like neon and the rest of the noble gases. Electronically, helium looks like beryllium and magnesium, the alkaline earth metals in Group 2.
A 2020 crystallographic analysis pointed out that helium shares more with the alkaline earths than is commonly acknowledged. Beyond the analogous electron configuration, solid helium adopts the same hexagonal close-packed crystal structure as beryllium and magnesium, a structural similarity that reflects the bulk material rather than isolated atoms.1PubMed Central. Helium’s placement in the Periodic Table from a crystal structure viewpoint Some periodic table designs therefore move helium to Group 2, sitting directly above beryllium. This is not a fringe idea; it has been seriously discussed in chemistry education circles for years. Most chemists still prefer helium above neon because the chemical behavior matters more to everyday use of the table, but the structural argument has real merit.
None of this changes helium’s classification as a nonmetal. Beryllium and magnesium are metals, but helium’s ionization energy is so vastly higher that it behaves nothing like them chemically. The debate is about where to put helium on the chart, not about what kind of element it is.
When Pressure Changes Everything
At the pressures and temperatures found on your kitchen counter or even deep underground, helium will always be a nonmetal. But physics does not stop at familiar conditions. Inside giant planets and in high-pressure laboratory experiments, materials can be forced into states that would seem impossible at the surface. Helium is no exception.
The basic idea is that if you compress any insulating material enough, you force its atoms so close together that their electron clouds overlap and the electrons become free to move. That is the definition of a metal in physics terms: a material whose electrons can flow freely and conduct electricity. For most elements, this transition happens at pressures that are extreme but achievable in a lab. For helium, the pressures required are staggering.
First-principles computational studies have explored what it would take to metallize solid helium at zero temperature. One set of diffusion quantum Monte Carlo calculations predicted the band gap would close at a density of about 21 grams per cubic centimeter and a pressure of roughly 25.7 terapascals.2PubMed. First-principles studies of the metallization and the equation of state of solid helium To put that in perspective, the pressure at Earth’s core is about 0.00036 terapascals. You would need pressures tens of thousands of times greater than those at the center of our planet to turn cold solid helium metallic. A later study that included the effects of atomic vibrations pushed the estimate even higher, to about 32.9 terapascals at absolute zero.3PubMed. Electron-phonon coupling and the metallization of solid helium at terapascal pressures The vibrations of the atoms in the lattice actually widen the band gap, making metallization harder to achieve than simpler calculations suggest.
These are not pressures anyone can produce in a laboratory today. They exist in the theoretical realm for now, predicted by computer simulations that model how electrons and nuclei interact under compression. But the predictions are remarkably consistent across different computational methods, and they tell us something important: helium is not inherently immune to becoming metallic. It is just the hardest element to push over that line.
Heat as an Alternative Route
Cold compression is not the only path to metallic helium. Temperature helps. When you heat a compressed material, the thermal energy jostles the atoms into disordered configurations, and those disordered arrangements can close the electronic band gap at lower pressures than would be needed in the cold.
Simulations of fluid helium at conditions resembling the deep interiors of giant planets found that the band gap closes at around 20,000 kelvin at a density roughly half that needed for zero-temperature metallization.4Proceedings of the National Academy of Sciences. Fluid helium at conditions of giant planetary interiors That is still an extraordinary temperature, hotter than the surface of the Sun by a factor of three, but the required density drops substantially when heat is involved. Another simulation study identified the nonmetal-to-metal transition in fluid helium at a density of about 1 gram per cubic centimeter, which is dense but not inconceivable compared to the 21 grams per cubic centimeter needed in the cold.5PubMed. Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium
Experimental work has started to probe this territory. By combining diamond-anvil cells with laser-driven shock waves, researchers have compressed helium samples to densities up to 1.5 grams per cubic centimeter at temperatures reaching 60,000 kelvin. Optical measurements at those conditions showed that electronic conduction in helium was temperature-activated, behaving like a semiconductor rather than a full metal. A fit to the data suggested that the mobility gap closes and helium becomes metallic above roughly 1.9 grams per cubic centimeter.6PubMed. Insulator-to-conducting transition in dense fluid helium Separate computational work on warm dense helium found that the band gap closure comes from both pressure-driven broadening of the valence band and temperature-induced shifts of the conduction band to lower energies.7Scientific Reports. Revisiting metallization boundary of warm dense helium in a wide ρ-T regime from ab initio study
So the experimental picture is not yet complete, but the trajectory is clear: at sufficiently extreme conditions, helium transitions from insulator to semiconductor to metal. The transition is gradual, not a sharp switch, and it happens at conditions that make most laboratory equipment irrelevant.
Why the Pressures Are So Extreme
You might wonder why helium holds out so stubbornly against metallization compared to other elements. Hydrogen, for instance, is predicted to metallize at pressures in the hundreds of gigapascals, which is extreme but within the reach of modern diamond-anvil experiments. Helium requires pressures roughly a hundred times higher. The reason comes down to its electronic structure. Helium’s two electrons sit in the lowest possible energy state, the 1s orbital, and they are bound extremely tightly to the nucleus. There is no second shell of electrons to push around, no partially filled orbitals waiting to overlap with neighbors. To force helium’s electrons into a conducting state, you have to overcome that immense binding energy, and that takes enormous compression.
The filled 1s shell also means helium has no chemical incentive to share or donate electrons. This is why helium is both the most chemically inert element and the hardest to metallize: both properties trace back to the same electronic stubbornness. Other noble gases like xenon and krypton metallize at far lower pressures because their outer electrons sit in higher-energy shells that are easier to delocalize.
Helium Inside Giant Planets
The question of metallic helium is not purely academic. Inside Jupiter and Saturn, hydrogen and helium exist together at pressures and temperatures where hydrogen has already become a metallic fluid. What helium does in that environment matters enormously for understanding how these planets work.
At the pressures found deep within Jupiter, hydrogen transitions from a molecular insulating fluid to a metallic one. Helium, being harder to metallize, does not make the same transition at the same point. This mismatch creates a problem: metallic hydrogen and insulating helium do not mix well. Simulations and experiments have shown that at megabar pressures and temperatures along the planetary interior profile, hydrogen and helium become immiscible, meaning they separate like oil and water.8PubMed. Demixing of hydrogen and helium at megabar pressures The researchers found a close relationship between this phase separation and the continuous nonmetal-to-metal transition in hydrogen, suggesting that it is precisely hydrogen’s metallization that triggers the demixing.
Laser-driven shock compression experiments on hydrogen-helium mixtures at Jupiter-like conditions have confirmed this immiscibility. A clear discontinuous change in sample reflectivity indicated a region of immiscibility that ends above 150 gigapascals at about 10,200 kelvin, with a more subtle change above 93 gigapascals at around 4,700 kelvin.9PubMed. Evidence of hydrogen-helium immiscibility at Jupiter-interior conditions When mapped onto Jupiter’s interior pressure and temperature profile, these constraints suggest that hydrogen-helium phase separation affects roughly 15 percent of the planet’s radius.
The practical consequence is “helium rain.” When helium becomes insoluble in the surrounding metallic hydrogen, it forms dense droplets that sink toward the planet’s core. This process releases gravitational energy as heat, which helps explain why Saturn radiates more heat than it receives from the Sun. Models that incorporate helium rain can explain the thermal evolution of both Jupiter and Saturn with a single consistent framework.10The Astrophysical Journal. Evidence for a Dichotomy in the Interior Structures of Jupiter and Saturn from Helium Phase Separation The limited solubility of helium in metallic hydrogen leads to helium-rich droplets raining out on timescales short compared to convective redistribution, creating nontrivial helium distributions throughout the planets’ interiors.
How Helium Alters Hydrogen’s Metallization
The relationship between helium and hydrogen under extreme conditions runs both ways. Helium does not just passively fail to metallize while hydrogen transitions around it. The presence of helium actively interferes with hydrogen’s metallization process.
Simulations of hydrogen-helium mixtures at planetary conditions have shown that even a small amount of helium strengthens the molecular bonds between hydrogen atoms, delaying hydrogen’s dissociation by roughly 250 kelvin or more depending on the helium fraction and pressure.11The Journal of Physical Chemistry Letters. Direct Simulations of H–He Mixtures at Planetary Interior Conditions: Demixing, Insulator–Metal Transition and Miscibility Boundaries The same study found that helium can reduce the electrical and thermal conductivity of the mixture by a factor of two to several thousand compared to pure hydrogen. That is a massive effect. It means that the metallic hydrogen ocean inside Jupiter does not conduct electricity nearly as well as models based on pure hydrogen would predict, which has real implications for understanding the planet’s magnetic field and how heat moves through its interior.
Quantum Monte Carlo simulations of a hydrogen-helium mixture with a protosolar composition (the roughly 75/25 hydrogen-to-helium ratio found in the Sun and gas giants) confirmed the crucial influence of helium on hydrogen’s metallization pressure.12PubMed. Phase Diagram of Hydrogen and a Hydrogen-Helium Mixture at Planetary Conditions by Quantum Monte Carlo Simulations The metallization happens at a higher pressure in the mixture than in pure hydrogen, shifted by helium’s stabilizing influence on molecular bonds. The upshot is that planetary models cannot simply treat the hydrogen-helium interior as pure hydrogen with some inert filler. The helium is chemically passive in everyday life, but at megabar pressures it is an active participant in the physics.
Pushing the Experimental Frontier
Most of what we know about helium’s behavior at extreme pressures comes from computer simulations. The experimental side is catching up, but slowly. The challenge is that the pressures needed to metallize pure helium are far beyond what any static compression device can achieve. Diamond-anvil cells top out in the hundreds of gigapascals; helium metallization requires tens of terapascals, a gap of roughly two orders of magnitude.
Dynamic compression using lasers offers a partial workaround. By pre-compressing helium samples in a diamond-anvil cell and then hitting them with powerful laser-driven shock waves, researchers have reached pressures up to 360 gigapascals, nearly doubling the previous experimental record for helium.13Physical Review Research. Shock compression of liquid helium to 360 GPa That is impressive, but still a tiny fraction of the terapascal pressures needed for full metallization of pure helium. The experiments are useful for benchmarking the equation of state and testing whether the computational predictions are on the right track, but they cannot yet reach the metallic state itself.
The warm-fluid experiments described earlier got closer to the transition by using both pressure and temperature, reaching conditions where helium showed semiconducting behavior and appeared to be approaching a metallic state. But even those heroic experiments stopped short of producing a fully metallic sample. For now, metallic helium remains a computationally predicted state that has not been directly observed in a pure helium sample, although all the indirect evidence points toward its reality.
Helium-Core White Dwarfs
Another setting where helium’s behavior under extreme density matters is inside white dwarf stars. Some white dwarfs have cores composed primarily of helium rather than the more common carbon-oxygen mixture. These helium-core white dwarfs form in binary star systems where mass transfer strips the outer layers of a star before it can fuse helium into heavier elements.
Inside these objects, helium exists at densities where it is fully degenerate, meaning quantum mechanical effects dominate and the electrons form a gas that resists further compression through the Pauli exclusion principle. Under these conditions, heavier elements like carbon, nitrogen, and oxygen have the same charge-to-mass ratio as helium and experience no net force in the degenerate core, performing a random walk inward until they reach a constant mass fraction in diffusive equilibrium.14The Astrophysical Journal. Helium Core White Dwarfs in Cataclysmic Variables Hydrogen, being lighter, is pushed outward by the electric field in the degenerate gas.
The thermal and optical properties of these stars produce some counterintuitive results. Modeling of helium-core white dwarfs found that when the effective temperature drops below about 4,000 kelvin, the emergent spectrum actually becomes bluer rather than redder, the opposite of what other researchers had reported and the opposite of everyday intuition about cooling objects.15Monthly Notices of the Royal Astronomical Society. The ages and colours of cool helium-core white dwarf stars This happens because of the way helium’s opacity changes at low temperatures and extreme densities, a reminder that familiar elements can behave in deeply unfamiliar ways when the conditions are pushed far enough from normal.
In these stellar interiors, helium is not metallic in the band-theory sense, but the electron gas is degenerate and highly conductive. The boundary between “metal” and “degenerate matter” gets philosophically fuzzy in such environments, though physicists would not typically call the helium in a white dwarf metallic. It is a distinct quantum state of matter that shares some properties with metals, like electrical conductivity, while being fundamentally different in origin. The degenerate electron gas conducts because quantum statistics force electrons into high-energy states, not because atomic orbitals have overlapped into a conduction band.