Notable Isotopes 
36Ar [18 neutrons]
Abundance: 0.337%
Stable with 18 neutrons
37Ar [19 neutrons]Abundance: synthetic
Half life: 35 days [ Electron Capture ]
Decay Energy: unknown MeVDecays to 37Cl.
| 37Ar is created from the decay of 40Ca as a result of subsurface nuclear explosions. It has a half-llife of 35 days. |
38Ar [20 neutrons]Abundance: 0.063%
Stable with 20 neutrons
39Ar [21 neutrons]Abundance: synthetic
Half life: 269 years [ beta- ]
Decay Energy: 0.565 MeVDecays to 39K.
| In the Earth's atmosphere, 39Ar is made by cosmic ray activity, primarily with 40Ar. |
42K [24 neutrons]Abundance: synthetic
Half life: 32.9 years [ beta- ]
Decay Energy: 0.600 MeVDecays to 42K.