Helium (He)
Element 2 · Noble gas · Gas at 25 °C · Group 18, Period 1
Helium is the second most abundant element in the universe. Lighter than air, it lifts balloons and airships, and as an ultra-cold liquid it cools the superconducting magnets inside MRI scanners.
On the periodic table, helium sits in group 18 — the noble gases — in period 1. It is one of 7 elements classified as a noble gas, and at room temperature (25 °C) it is a gas — one of 11 elements that are gases at room temperature.
The noble gases of group 18 have completely filled outer electron shells, so they have almost no urge to react with anything. All the naturally occurring members are colorless, odorless, monatomic gases at room temperature. Their aloofness is useful in its own right — inert atmospheres, lighting, and cryogenics all rely on it.
Properties of helium
| Atomic number (Z) | 2 |
|---|---|
| Symbol | He |
| Atomic mass | 4.0026 |
| Category | Noble gas |
| Group | 18 |
| Period | 1 |
| Block | s-block |
| Metal / nonmetal | Nonmetal |
| State at 25 °C | Gas |
| Electron configuration | 1s2 |
| Electronegativity (Pauling) | — |
| Melting point | — |
| Boiling point | -268.93 °C (4.22 K / -452.07 °F) |
| Density | 0.000179 g/cm³ |
Position in the periodic table
Helium shares group 18 — the noble gases with Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn), Oganesson (Og). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.
Physical properties
As a gas at standard conditions, its density is 0.000179 g/cm³ — measured at STP like the other gaseous elements here.
Helium has no accepted Pauling electronegativity value, which is why that field shows a dash.
See also: the interactive periodic table, how to read the periodic table, the element families tour.