Argon (Ar) — Atomic Number 18
Argon is a chemical element with the symbol Ar and atomic number 18. It is a colourless, odourless noble gas — the most abundant noble gas in Earth’s atmosphere (~0.93%). The name comes from Greek argos (“inactive”).
At a Glance
| Property | Value |
|---|---|
| Symbol | Ar |
| Atomic Number (Z) | 18 |
| Mass Number (A) | 40 |
| Protons | 18 |
| Neutrons | 22 |
| Electrons | 18 |
| Electron Configuration | 2,8,8 |
| Group | 18 / 0 (Noble Gas) |
| Period | 3 |
| State at RTP | Colourless, odourless gas |
| Classification | Non-metal — Monatomic (Ar) |
Discovery
Discovered in 1894 by Lord Rayleigh and William Ramsay. Rayleigh noticed that nitrogen from air was consistently denser than nitrogen from chemical reactions — the difference turned out to be argon.
Properties & Uses
- Full outer shell (2,8,8) — chemically inert
- Obtained by fractional distillation of liquid air
- Used as an inert shielding gas in welding
- Fills light bulbs to prevent filament oxidation
- Double-glazed windows — argon fill for insulation
IGCSE Essentials
- Monatomic — Ar, not Ar₂
- Most abundant noble gas in air (~0.93%)
- Configuration 2,8,8 — the octet achieved by Cl⁻, K⁺, Ca²⁺
- Chemically inert — full outer shell