Argon
Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most common gas in the Earth's atmosphere, at 0.934% (9,340 ppmv), making it over twice as abundant as the next most common atmospheric gas, water vapor (which averages about 4000 ppmv, but varies greatly), and 23 times as abundant as the next most common non-condensing atmospheric gas, carbon dioxide (400 ppmv), and more than 500 times as abundant as the next most common noble gas, neon (18 ppmv).
At a glance
- Atomic number
- 18
- Atomic mass
- 39.9481 u
- Phase
- Gas
- Density
- 1.784 g/L
- Melting point
- 83.81 K
- Boiling point
- 87.302 K
- First ionization energy
- 1,520.6 kJ/mol
- Electronegativity
- Unavailable
- Electron affinity
- -96 kJ/mol
- Molar heat capacity
- Unavailable
- Period
- 3
- Appearance
- colorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Argon’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
10 is the [Ne] core total across two shells.
Preparing the orbital view…
18 protons22 neutrons
Argon-40: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FGas
At this temperature, argon is a gas.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
ArNeutral atom · no charge
A neutral argon atom. Removing its first electron takes 1,521 kJ/mol.
Argon’s outer shell holds 8 electrons. Electron 9 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 2.9× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 8, then 8, then 2 bars. Read backwards, that’s 2, 8, 8: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 8 electrons before the first big jump are argon’s valence electrons: a full outer shell. That’s why argon sits in group 18 with the other noble gases, which all have full outer shells.
- Even the first electron is expensive: 1,521 kJ/mol. A full outer shell holds its electrons tightly, so noble gases have the highest first ionization energy in their row of the table, part of why they rarely react.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 3 | 1,521 | – | Measured |
| 2 | 3 | 2,666 | 1.8 | Measured |
| 3 | 3 | 3,930 | 1.5 | Estimated |
| 4 | 3 | 5,749 | 1.5 | Estimated |
| 5 | 3 | 7,221 | 1.3 | Estimated |
| 6 | 3 | 8,808 | 1.2 | Measured |
| 7 | 3 | 12,004 | 1.4 | Estimated |
| 8 | 3 | 13,841 | 1.2 | Estimated |
| 9 | 2 | 40,775 | 2.9 | Estimated |
| 10 | 2 | 46,290 | 1.1 | Estimated |
| 11 | 2 | 52,141 | 1.1 | Estimated |
| 12 | 2 | 59,724 | 1.1 | Estimated |
| 13 | 2 | 66,141 | 1.1 | Estimated |
| 14 | 2 | 72,859 | 1.1 | Estimated |
| 15 | 2 | 82,543 | 1.1 | Estimated |
| 16 | 2 | 88,610 | 1.1 | Calculated |
| 17 | 1 | 397,584 | 4.5 | Calculated |
| 18 | 1 | 427,066 | 1.1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
A fingerprint made of light
Argon glows lavender in a tube. Mixed with a little mercury and inside a coated tube, it lights many of the “neon” signs that aren’t red.
Electrons race through the tube and crash into argon atoms, knocking the atoms’ own electrons up to higher energy levels. As those fall back, the atoms give off light only at the exact wavelengths below, so each gas glows its own colour.
The five gases’ visible lines side by side: each pattern is a fingerprint. Choose one to see it in detail below.
Click a line to see where it comes from.
696.5 nmred
An electron falls from the 4p level down to the 4s level. It loses 1.78 eV of energyAn electronvolt (eV) is a tiny unit of energy used for atoms: the energy one electron gains crossing 1 volt. A particle of visible light carries between about 1.7 eV (red) and 3.3 eV (violet). and gives it off as one particle of red light. Bigger drops give bluer light.
The strongest visible lines of neutral argon from the NIST Atomic Spectra Database. Line brightness follows NIST’s relative intensities, which only compare lines of the same element. Colours are approximate, and lamp colours are as they look to the eye.
How does it compare?
Density
0.00178 g/cm³7th lowest of 99
Melting point
83.8 K6th lowest of 96
Boiling point
87.3 K6th lowest of 97
Electronegativity
Argon forms almost no chemical bonds, so it has no Pauling electronegativity.
First ionization energy
1,521 kJ/mol4th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- ArgonRoyal Society of Chemistry
- ArgonWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database
- Spectral linesNIST Atomic Spectra Database