Chemical element · reference page

18Ar

Argon

Argon is element 18 (Ar), located in period 3, group 18 and the p block. At room temperature its reference phase is gas. Its electron configuration is [Ne] 3s2 3p6.

Atomic number18

Standard atomic weight[39.792, 39.963]

Group18

Period3

Blockp

Electron configuration[Ne] 3s2 3p6

Electronic structure

What controls its chemistry

The outer electron shell is filled, which strongly reduces the tendency to gain, lose or share electrons. This is the central reason noble gases are much less reactive than neighbouring groups.

The heavier noble gases are not completely inert: under suitable conditions some form compounds, especially with highly electronegative elements. The group trend is therefore low reactivity, not an absolute prohibition on chemistry.

Chemical forms

Compounds, ions and materials

Argon 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 1.784 g/L. The melting point is 83.81 K (-189.3 °C). The boiling point is 87.3 K (-185.8 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where argon is found and used

In practice, argon is encountered in welding, lighting and inert atmospheres.

In practice

Examples of real uses

Welding

Argon is used as a shielding gas or alloy component to control oxidation and metal quality during welding.

Lighting

Argon is used because atoms or compounds of this element interact with light in a useful and reproducible way.

Inert atmospheres

Argon, a low-reactivity gas, is used to protect hot or sensitive materials from air, moisture or oxidation.

Periodic position

What its neighbours tell us

Argon is in period 3 and group 18. In the neighbourhood shown here, chlorine is immediately to its left in the same period; neon is above it and krypton 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 · 8 · 8

Density1.784 g/L

Melting point83.81 K (-189.3 °C)

Boiling point87.3 K (-185.8 °C)

Pauling electronegativity—

First ionisation energy1 521 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 argon is associated with Lord Rayleigh. 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 given as the interval [39.792, 39.963]. Natural samples can differ slightly in isotope proportions, so a single universal decimal value would hide real variation between materials.

Argon 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.