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Element 18 · Argon (Ar)

Argon Bohr model: diagram and electrons per shell

The argon Bohr model shows a argon atom (Ar) as a small nucleus with its electrons circling it in 3 shells: 2 in shell K, 8 in shell L and 8 in shell M. This page has a rotating 3D argon Bohr model, a flat diagram, a table of the electrons in each shell and the steps to draw it yourself.

Rotate the argon Bohr model in 3D

Drag to orbit, scroll to zoom.

This is a Bohr model of argon (Ar), a simplified picture of a single argon atom. Its nucleus holds 18 protons, and a neutral argon atom has 18 electrons, arranged around it in 3 shells (2 in shell K, 8 in shell L and 8 in shell M).

Ar K · 21s2 L · 82s2 2p6 M · 83s2 3p6

Argon Bohr model diagram

Bohr model diagram of argon: a nucleus of 18 protons with 18 electrons on 3 shells, 2 in shell K, 8 in shell L and 8 in shell M, each ring labelled with its shell letter.
The argon Bohr model drawn flat: each ring is a shell, labelled with its letter, and every dot is an electron (2 in shell K, 8 in shell L and 8 in shell M).

Argon electrons per shell

A neutral argon atom has 18 electrons, one for each of its 18 protons. They fill the shells from the nucleus outwards: 2 in shell K, 8 in shell L and 8 in shell M.

Electrons in each shell of the argon Bohr model
ShellShell number (n)ElectronsMost it can hold (2n²)Subshells
K1221s2
L2882s2 2p6
M38183s2 3p6

How the argon Bohr model builds up, shell by shell

Watch the argon Bohr model come together: the nucleus first, then each shell in turn, from the innermost outwards, with its electrons joining the ring. The legend names every shell and counts its electrons, so you can read off the arrangement (2 in shell K, 8 in shell L and 8 in shell M) as it forms.

Animation of the argon Bohr model being built: a nucleus of 18 protons appears first, then each shell grows in around it and takes its electrons, giving 18 electrons on 3 shells (2 in shell K, 8 in shell L and 8 in shell M).
The argon Bohr model built up in six seconds: the nucleus first, then each shell with its electrons, ending with 2 in shell K, 8 in shell L and 8 in shell M. Protons are red, neutrons yellow and electrons blue.

How to draw the argon Bohr model

  1. Find the atomic number. Argon has atomic number 18, so a neutral atom has 18 protons and 18 electrons.
  2. Draw the nucleus. Put a circle in the centre and write Ar in it. The 18 protons sit there with the neutrons.
  3. Draw the shells. Sketch 3 shells as rings around the nucleus. A shell holds at most 2n² electrons: 2 in the first, 8 in the second, 18 in the third and 32 in the fourth.
  4. Place the electrons. Fill from the inside out: 2 in shell K, 8 in shell L and 8 in shell M.
  5. Check the total. 2 + 8 + 8 = 18, the atomic number, so every electron is accounted for.

Atomic data for the argon Bohr model

The numbers behind the argon Bohr model, grouped by topic. Radii are in picometres (pm) and energies in kilojoules per mole (kJ/mol).

Identity

Symbol
Ar
Atomic number
18
Atomic mass
39.9481 u
Family
Noble Gases
Position
group 18, period 3
Block
p-block

Electron configuration

Full
1s2 2s2 2p6 3s2 3p6
Shorthand
[Ne] 3s2 3p6
Electrons per shell
2, 8, 8
Outer-shell electrons
8 in shell M

Ionisation energies

I₁
1520.6
I₂
2665.8
I₃
3931
I₄
5771
I₅
7238
I₆
8781
I₇
11995
I₈
13842

Energy to remove each electron in turn (kJ/mol). A sharp jump means the next electron comes from an inner shell.

Atomic radii

Calculated (pm)
71
Empirical (pm)
71
Covalent (pm)
96
Van der Waals (pm)
188

Electronegativity and electron affinity

Electron affinity (kJ/mol)
-96

Physical properties, abundance and history of argon

Bulk properties of argon, how common it is, who discovered it and some of its compounds.

Physical properties

Phase at room conditions
Gas
Appearance
Colorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field
Melting point
-189.34°C (83.81 K)
Boiling point
-185.85°C (87.302 K)
Density
0.001784 g/cm³
Thermal conductivity (W/m·K)
0.018
Specific heat (J/g·K)
0.52
Heat of vaporisation (kJ/mol)
6.52

Abundance

Earth's crust (mg/kg)
3.5
Oceans (mg/L)
0.45

Discovery

Discovered by
Lord Rayleigh
Year
1894

Common compounds

  • Argon Ar
  • Argon fluoride ArF-
  • Argon-39 Ar
  • Argon-41 Ar
  • Argon-40Ar Ar
  • Argon-36 Ar
  • Argon-37 Ar
  • Argon hydrogen bromide ArBrH

Compounds listed in the PubChem database.

Main isotopes of argon

Isotopes of argon share the same 18 protons but differ in neutrons. Natural isotopes come first, then the longest-lived others.

Main isotopes of argon: mass number, abundance, half-life and decay
Mass numberNatural abundanceHalf-lifeDecay
360.334 %stable2β⁺
37–35.011 dEC
380.063 %stable–
39–268 yβ⁻
4099.603 %stable–
42–32.9 yβ⁻

α = alpha decay, β⁻ = beta-minus, β⁺ = beta-plus, EC = electron capture, SF = spontaneous fission, p / n = proton / neutron emission.

More about argon