Element 18 · Noble gas
Argon (Ar) — Element 18
Greek argos — idle
Argon (Ar) is a noble gas in Period 3, Group 18 of the periodic table with atomic number 18, standard atomic weight 39.948 u, and ground-state electron configuration [Ne] 3s² 3p⁶. At standard temperature and pressure it is classified as a gas with 0 valence electrons and common oxidation states 0.
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What are the atomic properties and physical constants of Argon?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 18
- Standard atomic mass
- 39.948 u
- Chemical family
- Noble gas
- Ground-state configuration
- [Ne] 3s² 3p⁶
- Electrons per shell
- 2, 8, 8
- Valence electrons
- 0
- Common ion charges / oxidation states
- 0
- Phase at STP
- Gas
- Pauling electronegativity
- —
- First ionization energy
- 1,521 kJ/mol
- Atomic radius
- 71 pm
- Melting point
- -189 °C
- Boiling point
- -186 °C
- Discovery year
- 1894
- Table coordinate
- Period 3, Group 18
- Radioactivity
- Has stable isotope(s)
How are electrons arranged across Argon’s shells?
A neutral Argon atom contains 18 electrons distributed across 3 occupied principal energy shells ([Ne] 3s² 3p⁶). The table below compares Argon’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Ar | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 8 e⁻ | 18 e⁻ |
Frequently Asked Questions about Argon (Ar)
What is the atomic mass and atomic number of Argon (Ar)?
Argon (Ar) has atomic number Z = 18 (18 protons in its nucleus) and an IUPAC standard atomic weight of 39.948 u. It sits in Period 3, Group 18 and is classified as a noble gas.
What is the ground-state electron configuration of Argon?
The neutral ground-state electron configuration of Argon (Z = 18) is [Ne] 3s² 3p⁶. Across its 3 occupied principal shells (n = 1 to n = 3), the electrons per shell are 2, 8, 8, with 0 valence electrons participating in bonding.
What are the common ion charges and electronegativity of Argon?
Argon commonly forms 0 oxidation states or ion charges in compounds. Its Pauling electronegativity is — and its first ionization energy is 1,521 kJ/mol.
When was Argon discovered and what is it used for?
Argon was discovered in 1894 (Greek argos — idle). Welding, light bulbs, windows
How does Argon compare to neighboring elements?
Other noble gas elements: