---
title: "Argon (Ar) — element 18"
description: "Argon is element 18, symbol Ar, atomic weight 39.95. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/argon/
site: "At One Place"
---

# Argon (Ar)

**Argon — element 18: Ar**

39.95 u · Noble gas · period 3, group 18

Argon is the third most abundant gas in the atmosphere at 0.93%, more common than carbon dioxide, and almost all of it is argon-40 produced by potassium-40 decaying inside rocks over billions of years — the atmosphere of a planet with less potassium would have very little argon. Rayleigh found it by noticing that nitrogen from the air was consistently 0.5% denser than nitrogen made chemically, a discrepancy he refused to ignore, and the pair got a Nobel Prize for chasing it down. The name means "the lazy one".

## Properties

- **Atomic number:** 18
- **Symbol:** Ar
- **Standard atomic weight:** 39.95 u
- **Category:** Noble gas
- **Group:** 18
- **Period:** 3
- **Block:** p-block
- **Electron configuration:** [Ne] 3s² 3p⁶
- **Electrons per shell:** 2, 8, 8
- **Melting point:** 83.8 K — -189.3 °C — -308.8 °F
- **Boiling point:** 87.3 K — -185.8 °C — -302.5 °F
- **Density:** 0.0017837 g/cm³
- **Oxidation states:** 0
- **Discovered:** 1894 by Lord Rayleigh and William Ramsay

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶
- **Noble gas shorthand:** [Ne] 3s² 3p⁶
- **Shells:** K: 2 · L: 8 · M: 8

## What it is used for

- Shielding gas for MIG and TIG welding
- The fill gas between double-glazing panes
- Blanketing molten metal and reactive chemicals
- Filling incandescent and halogen bulbs
- The carrier gas in gas chromatography and plasma spectrometry

## Hazards

- Asphyxiant; denser than air and collects in pits, tanks and ship holds

> IUPAC publishes an interval, [39.792, 39.963]; 39.95 is the conventional value, and the spread is unusually wide because argon from different geological sources carries different amounts of radiogenic argon-40. Density is the gas at 0 °C and 101.325 kPa.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 16 | Sulfur | S | 32.06 |
| 17 | Chlorine | Cl | 35.45 |
| 18 | Argon ← | Ar | 39.95 |
| 19 | Potassium | K | 39.098 |
| 20 | Calcium | Ca | 40.078 |

## Group 18

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Argon sits in group 18 with helium, neon, krypton, xenon, radon, oganesson.

## Common questions

### What is the atomic number of argon?

18. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for argon?

Ar.

### What is the electron configuration of argon?

1s² 2s² 2p⁶ 3s² 3p⁶, or [Ne] 3s² 3p⁶ in noble-gas shorthand.

### What is the melting point of argon?

83.8 K — -189.3 °C — -308.8 °F

## Related pages

- [Sulfur (S)](https://atoneplace.net/element/sulfur/)
- [Chlorine (Cl)](https://atoneplace.net/element/chlorine/)
- [Potassium (K)](https://atoneplace.net/element/potassium/)
- [Calcium (Ca)](https://atoneplace.net/element/calcium/)
- [Helium (He)](https://atoneplace.net/element/helium/)
- [Neon (Ne)](https://atoneplace.net/element/neon/)
- [Krypton (Kr)](https://atoneplace.net/element/krypton/)
- [Xenon (Xe)](https://atoneplace.net/element/xenon/)
- [Radon (Rn)](https://atoneplace.net/element/radon/)
- [Oganesson (Og)](https://atoneplace.net/element/oganesson/)

## Sources

- Official vendor product documentation — At One Place (/about/)