018Noble gas · Group 18 · Period 3
Argon
Argon (Ar) is element 18, a noble gas in the p-block — group 18, period 3. It is a gas at room temperature with 3 electron shells (2, 8, 8) and an atomic mass of 39.9481 u.
- Atomic mass
- 39.95 u
- Phase · STP
- gas
- Melting
- 84 K · -189 °C
- Boiling
- 87 K · -186 °C
- Electronegativity
- —
- Density
- 1.784 g/cm³
Photograph — Jurii, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons, source: https://images-of-elements.com/argon.php
Emission spectrum
The visible light Argon emits when energised — its unique spectral fingerprint.
Identity
- Atomic number
- 18
- Standard atomic weight
- 39.95 u
- Category
- Noble gas
- Group
- 18 · Noble gases
- Period
- 3
- Block
- p-block
- Oxidation states
- 0
Physical properties
- Melting point
- 84 K · -189 °C
- Boiling point
- 87 K · -186 °C
- Density
- 1.784 g/cm³
- Molar heat capacity
- —
- Atomic radius
- 188 pm
- Electron affinity
- -96 kJ/mol
Atomic structure
18protons
22neutrons
18electrons
8valence e⁻
- Noble-gas notation
- [Ne] 3s2 3p6
- Full configuration
- 1s2 2s2 2p6 3s2 3p6
- Electrons per shell
- 2, 8, 8
Periodic trends
Where Argon ranks across all 118 elements.
Ionization energies
Energy to remove each successive electron (kJ/mol).
1.5k1
2.7k2
3.9k3
5.8k4
7.2k5
8.8k6
12.0k7
13.8k8
40.8k9
46.2k10
Successive ionization energies (kJ/mol, log scale).
Discovery
- Discovered
- 1894
- Discovered by
- Lord Rayleigh
- Named by
- —
- Appearance
- colorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field