Argon (Ar)
Argon is element 18, a noble gas in group 18, period 3. A neutral atom has 18 protons, 22 neutrons and 18 electrons, 8 of them valence electrons.
Noble gas- Atomic number
- 18
- Atomic mass
- 39.9 u
- Valence electrons
- 8
- Protons · neutrons
- 18 · 22
- Electrons per shell
- 2, 8, 8
- At room temperature
- gas
Bohr model of argon
The Bohr model draws argon as a nucleus of 18 protons and 22 neutrons, with its 18 electrons on 3 circular shells: 2 in shell 1, 8 in shell 2, 8 in shell 3. Press Build the atom to watch the shells fill from the inside out. The outer ring (the M shell) is dashed: the electrons there are the ones that take part in chemistry.
Real electrons don't travel in neat circles. They live in fuzzy orbitals, shown below, but the Bohr picture is the quickest way to count shells and valence electrons.
Electron configuration of argon
1s² 2s² 2p⁶ 3s² 3p⁶
Short form (noble gas notation): [Ne] 3s² 3p⁶. The [Ne] stands for the electrons of neon, the noble gas before it.
How many valence electrons does argon have?
Argon has 8 valence electrons.
Argon is in group 18, and for groups 13–18 you subtract 10 from the group number: 18 − 10 = 8. A full outer shell of eight makes it a noble gas that rarely reacts.
Protons, neutrons and electrons
| Protons | 18 | equal to the atomic number |
|---|---|---|
| Electrons | 18 | equal to the protons in a neutral atom |
| Neutrons | 22 | mass number 40 − 18 protons |
Atoms of the same element can have different numbers of neutrons (isotopes). The count above uses the rounded atomic mass, the way it's taught in class.
Argon properties
| Symbol | Ar |
|---|---|
| Category | Noble gas |
| Group · period · block | 18 · 3 · p |
| Atomic mass | 39.9 u |
| State at 25 °C | gas |
| Melting point | 83.8 K (-189 °C, -309 °F) |
| Boiling point | 87.3 K (-186 °C, -303 °F) |
| Density | 0.0018 g/cm³ |
| Electronegativity (Pauling) | — |
| Atomic radius | 188 pm |
| First ionization energy | 15.76 eV |
| Electron affinity | — |
| Oxidation states | 0 |
| Common ion charge | 0 |
| Discovered | 1894 by Lord Rayleigh |
Where argon sits on the periodic table
Group 18: Helium, Neon, Krypton, Xenon, Radon, Oganesson.
Questions about argon
How many valence electrons does argon have?
Argon has 8 valence electrons.
How many protons, neutrons and electrons does argon have?
A neutral argon atom has 18 protons and 18 electrons. Its most common form has about 22 neutrons (mass number 40 − 18 protons).
What is the electron configuration of argon?
1s² 2s² 2p⁶ 3s² 3p⁶, written in short form as [Ne] 3s² 3p⁶.
What does the Bohr model of argon look like?
A nucleus of 18 protons and 22 neutrons, circled by 3 electron shells holding 2, 8, 8 electrons.
What is the atomic number of argon?
18. The atomic number is the number of protons in the nucleus, and it is what makes an atom argon.