In nature, 3 isotopes of potassium are present: potassium 39, 40, 41. All have the same number of protons (19 in number, and electrons, which makes them the chemical element potassium), potassium 39 and 41 are stable/non-radioactive, potassium 40 is radioactive, a positron emitter with a half-life of 1.25 billion years.There are also another 22 radioactive potassium isotopes, but they have such a short half-life that they do not occur in the environment, because all the material produced has disintegrated for a long time.
Of course, the question remains why there is radioactive potassium in the first place and why only the two isotopes with atomic masses 39 and 41 are stable?To do this, you have to get into nuclear physics a little bit…
As mentioned above, the element potassium has 19 protons at its core.Now, similar to the atomic electron shell, there are particularly stable protons/neutron configurations for atomic nuclei. In the case of the electron shell, these are the noble gas configuration 鈥?Wikipedia, in the atomic nucleus these stable configurations are called ” Magic Number(Physics) 鈥?Wikipedia” in the protons/neutron numbers 2, 8, 20, 28, 50, 82, 126.Furthermore, it is shown that cores with even-numbered protons/neutron numbers are more stable than cores with uneven numbers. Potassium “misses” a proton to a magic configuration with 20 protons (which would then be calcium…), at 39K there is a magic number of neutrons, which makes the nucleus very stable and causes this to be the most common potassium isotope (93.7%) Is. At 41K, there is a “magical” neutron configuration plus a neutron eperator in which the neutrons stabilize each other.
At 40K there is now a “lonely” neutron over a closed neutron shell and there are two “attractive” neighbor isotopes: Argon 40 with 18 protons and 20 magical neutrons and Calcium 40 with 20 protons and 18 neutrons.This “voltage ratio” in the 40K leads to the transition/disintegration to the stable argon/calcium isotopes being energetically advantageous (albeit only very, very little) and thus occurring with a certain, small probability. The decay to calcium is by beta decay (conversion of the neutron into a proton plus electron and emission of the electron from the nucleus) can be carried out more easily than the decay to 40Ar, since there either an electron from the electron shell merges with a proton with a proton or spontaneously has to form an electron/positron pair in the nucleus and the electron merges with a proton into a neutron.
Similar considerations apply to the other radioactive isotopes of potassium, but the imbalances between stable nucleon configuration and current configuration are growing, leading to greater decay/conversion probabilities, orshorter half-life.