Notable Isotopes 
39K [20 neutrons]
Abundance: 93.26%
Stable with 20 neutrons
40K [21 neutrons]Abundance: 0.012%
Half life: 1.277 x 109 years [ beta- ]
Decay Energy: 1.311 MeVDecays to 40Ca.
Half life: 1.277 x 109 years [ Electron Capture ]
Decay Energy: 1.505 MeVDecays to 40Ar.
Half life: 1.277 x 109 years [ beta+ ]
Decay Energy: 1.505 MeVDecays to 40Ar.
| In healthy animals and people, 40K represents the largest source of radioactivity, greater even then 14C. In a human body of 70 kg mass, about 170,000 nuclei of 40K decay per second. |
41K [22 neutrons]Abundance: 6.73%
Stable with 22 neutrons