Helium
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among all the elements.
At a glance
- Atomic number
- 2
- Atomic mass
- 4.002602 u
- Phase
- Gas
- Density
- 0.1786 g/L
- Melting point
- 0.95 K
- Boiling point
- 4.222 K
- First ionization energy
- 2,372.3 kJ/mol
- Electronegativity
- Unavailable
- Electron affinity
- -48 kJ/mol
- Molar heat capacity
- Unavailable
- Period
- 1
- Appearance
- colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Helium’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
Preparing the orbital view…
2 protons2 neutrons
Helium-4: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FGas
At this temperature, helium is a gas.
Helium is the only element that never freezes at normal pressure, even close to absolute zero. Solid helium needs about 25 times atmospheric pressure.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
HeNeutral atom · no charge
A neutral helium atom. Removing its first electron takes 2,372 kJ/mol.
Helium has a single shell, so there is no inner-shell jump. The second electron still takes 2.2× as much as the first: with no other electron pushing it away, the last one is held more tightly.
What does this tell us?
- Even the first electron is expensive: 2,372 kJ/mol. A full outer shell holds its electrons tightly, so noble gases have the highest first ionization energy in their row of the table, part of why they rarely react.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 1 | 2,372 | – | Measured |
| 2 | 1 | 5,251 | 2.2 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
A fingerprint made of light
In 1868, astronomers watching a solar eclipse saw a yellow line at 587.6 nm that matched no element known on Earth. It was named helium, after Helios, the Greek word for the Sun, and wasn’t found on Earth until 1895.
Electrons race through the tube and crash into helium atoms, knocking the atoms’ own electrons up to higher energy levels. As those fall back, the atoms give off light only at the exact wavelengths below, so each gas glows its own colour.
The five gases’ visible lines side by side: each pattern is a fingerprint. Choose one to see it in detail below.
Click a line to see where it comes from.
388.9 nmviolet
An electron falls from the 3p level down to the 2s level. It loses 3.19 eV of energyAn electronvolt (eV) is a tiny unit of energy used for atoms: the energy one electron gains crossing 1 volt. A particle of visible light carries between about 1.7 eV (red) and 3.3 eV (violet). and gives it off as one particle of violet light. Bigger drops give bluer light.
The strongest visible lines of neutral helium from the NIST Atomic Spectra Database. Line brightness follows NIST’s relative intensities, which only compare lines of the same element. Colours are approximate, and lamp colours are as they look to the eye.
How does it compare?
Density
0.000179 g/cm³2nd lowest of 99
Melting point
Helium never freezes at normal pressure, so it has no melting point.
Boiling point
4.22 KThe lowest boiling point of 97 elements
Electronegativity
Helium forms almost no chemical bonds, so it has no Pauling electronegativity.
First ionization energy
2,372 kJ/molThe hardest to ionize of 99 elements
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- HeliumRoyal Society of Chemistry
- HeliumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database
- Spectral linesNIST Atomic Spectra Database