Argon
Argon is a chemical element with the symbol Ar and atomic number 18. It has a standard atomic weight of 39.95, sits in period 3 and group 18, and is classified as a noble gas.
Properties
| Symbol | Ar |
|---|---|
| Atomic number | 18 |
| Standard atomic weight | 39.95 |
| Category | Noble gas |
| Group | 18 |
| Period | 3 |
| Electron configuration | [Ne] 3s2 3p6 |
| Outer-shell electrons | 8 |
The figure of 39.95 is a conventional value, because IUPAC publishes an interval for this element rather than a single weight. For elements like this one the fourth digit carries real uncertainty, because the isotopic composition varies measurably between natural sources — so quoting eight digits would overstate what is known.
Compounds containing argon
1 of the compounds in this reference contain Ar, listed by the fraction of their mass it makes up.
| Compound | Formula | Atoms | % by mass |
|---|---|---|---|
| argon | Ar | 1 | 100.0% |
Calculate with argon
- Molar mass of any formula containing it
- Electron configuration, including ions
- Effective nuclear charge by Slater's rules
- Average atomic mass from isotope abundances
Group 18
Atomic weights are the IUPAC 2021 standard values, abridged to the conventional five significant figures. Group, period, category and electron configuration are derived from the atomic number rather than stored, so they cannot drift out of step with it.