He · Element 2 · Noble gas
Inert, weightless and permanently escaping: helium is the only element that will not freeze at ordinary pressure, no matter how cold you make it.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where helium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All helium atoms have the same number of protons, 2, and differ only in their number of neutrons: those variants are its isotopes. The most common, He-4, has 2 neutrons and makes up 99.9998% of natural helium.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| He-3 | 1 | 0.0002% | Stable | — |
| He-4 most common | 2 | 99.9998% | Stable | — |
Here the classroom shortcut, rounded atomic mass (4.003) minus atomic number, gives the same answer: 2.
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 31 pm — that is 0.031 nm, so about 16.1 million of them side by side would span a millimetre.
Thermal range
Helium is liquid over a 3 K window, from 1 K to 4 K.
The story
Helium, named after Helios the Greek sun god, holds the unique distinction of being the only element discovered in space before Earth. First detected in the sun's spectrum during the 1868 solar eclipse, helium is the second most abundant element in the universe yet one of the rarest on Earth.
Helium exhibits remarkable properties that defy common experience. It's the only element that cannot be frozen into a solid at normal pressure - even at absolute zero (-273.15°C), helium remains liquid unless pressure exceeds 25 atmospheres. At 2.17K, helium-4 becomes a superfluid with zero viscosity, capable of flowing upward and through molecular-sized pores.
As the first noble gas in the periodic table, helium is nearly inert, although compounds such as Na₂He form under very high pressure. Its electron configuration (1s²) represents a completed shell, making it one of the least reactive elements. Helium compounds are almost unknown in nature (a 2019 study detected HeH⁺ in the nebula NGC 7027), though exotic helium hydride ions (HeH⁺) have been created in laboratory conditions.
Helium-4 (⁴He): 99.999863% abundance - produced by alpha decay of radioactive elements
Helium-3 (³He): 0.000137% abundance - extremely rare, potential fusion fuel worth $15,000 per gram
Applications
Handling
SIMPLE ASPHYXIANT - OXYGEN DISPLACEMENT HAZARD
Gas Leak:
Inhalation Incident:
Cryogenic Exposure:
Quick answers
Helium (symbol He) is element 2 on the periodic table, a noble gas in period 1, group 18. Inert, weightless and permanently escaping: helium is the only element that will not freeze at ordinary pressure, no matter how cold you make it. At room temperature it is a gas.
1s², giving 1 occupied shell with the distribution 2. Its outer shell holds 2 electrons, which is what sets its bonding behaviour.Helium melts at 1 K (-272.2 °C) and boils at 4.2 K (-268.9 °C).
The standard atomic weight of helium is 4.003 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common helium atom, He-4, has 2 neutrons: its mass number, 4, minus its atomic number, 2. It makes up 99.9998% of natural helium. The classroom shortcut, rounded atomic mass (4.003) minus atomic number, gives the same 2.
Helium is a gas at room temperature; at 0 °C and 1 atm its density is about 0.178 g/L, so it is lighter than air.
Helium was discovered in 1868 by Pierre Janssen & Norman Lockyer. Origin of the name: Greek helios, the Sun — found there before Earth.
Helium makes up about 0.52 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.