Chemical element · reference page

2He

Helium

Helium is element 2 (He), located in period 1, group 18 and the s block. At room temperature its reference phase is gas. Its electron configuration is 1s2.

Atomic number2

Standard atomic weight4.002602(2)

Group18

Period1

Blocks

Electron configuration1s2

Electronic structure

What controls its chemistry

Helium has a filled 1s shell. With no low-energy vacancy in its outer shell, neutral helium has very little tendency to form ordinary chemical bonds. That is the electronic reason for its exceptionally low reactivity.

Its weak interatomic attractions also help explain helium’s extremely low boiling point. Helium is therefore chemically simple but physically important in cryogenics and low-temperature science.

Chemical forms

Compounds, ions and materials

Helium is monatomic under ordinary conditions. Stable compounds are rare compared with neighbouring groups because the valence shell is filled; the chemistry is dominated by the free atoms and weak intermolecular interactions.

Physical behaviour

State and phase changes

The reference phase at room temperature is gas. The listed density is 0.1786 g/L. The melting point is —. The boiling point is 4.222 K (-268.9 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where helium is found and used

In practice, helium is encountered in balloons, medical imaging equipment and cryogenics.

In practice

Examples of real uses

Balloons

Helium provides lift while remaining non-flammable, which is why it is preferred to hydrogen for many balloons and airships.

Medical imaging equipment

Helium is used in imaging agents, detector materials or the high-field magnets and specialised components of imaging systems.

Cryogenics

Helium is used at very low temperatures as a coolant, working fluid or material whose low-temperature behaviour is useful.

Periodic position

What its neighbours tell us

Helium is in period 1 and group 18. In the neighbourhood shown here, neon is directly below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationnoble gas

Phase at room temperatureGas

Electron shells2

Density0.1786 g/L

Melting point—

Boiling point4.222 K (-268.9 °C)

Pauling electronegativity—

First ionisation energy2 372 kJ/mol

Electron affinity—

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of helium is associated with Pierre Janssen. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 4.002602(2). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Helium has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The gas is not necessarily chemically toxic, but it can displace oxygen in confined spaces. Ventilation and oxygen monitoring matter in bulk handling.