Helium (He)

Colorless gas

Helium is the second lightest element and the second most abundant in the universe, known for filling balloons and making voices squeaky, and a noble gas with a completely filled 1s² shell, helium has the lowest boiling point of any element at 4.22 K and remains liquid near absolute zero without solidifying under normal pressure.

Key metrics

Density1.79e-4g/cm³

Melting point0.95K

Boiling point4.22K

Electronegativity0Pauling

Discovery

1895discovered

Discovered by: William Ramsay
Reactivity profile: Inert
Reactivity: Low
Bonding: Covalent

Electron configuration

\textcolor{#ef4444}{1_{s}^{2}}

1s2

Wave function

\Psi_{\text{He}} \approx \frac{1}{\sqrt{2!}} \det \left| \chi_1(\mathbf{r}_1) \chi_2(\mathbf{r}_2) \right|

Probability density $|\Psi|^2$ across the He valence shell.

Crystal structure

Hexagonal Close-Packed

Lattice constantsa 1.00 · b 1.00 · c 1.63

Cell anglesα 90° · β 90° · γ 120°

Coordination number12

Atoms per cell2

Atomic packing factor0.74

Physical properties

Atomic radius: 31 pm
Ionization energy: 2372 kJ/mol
Electron affinity: 0 kJ/mol
Crystal structure: Hexagonal close-packed (hcp)

Oxidation states

Acid/base

Neutral

Strongly acidicStrongly basic

State at 25 °CGas

0.95 K4.22 K298 K

Emission spectrum

4 transitions

380450500550600650700780

VioletBlueGreenYellowRed

447.1nmHe IViolet40%

501.6nmHe ICyan50%

587.6nmHe I (D3)Yellow100%

667.8nmHe IRed70%

Dominant line 587.6 nmRange 447–668 nmTransitions 4