Question

What is the natural transmutation which forms most of the argon in air?

Answer 1

Potassium-40 decays to argon-40 via electron capture.

Explanation:

Potassium-40, `""^40"K"`, will decay to argon-40, `""^40"Ar"`, via electron capture.

Electron capture takes place when an electron from the inner shells of an atom is captured by the nucleus. The negatively charged electron will combine with a proton, which is a positively charged particle, to form a neutron.

In addition to this, an electron neutrino, `nu_e`, is being emitted.

So, if a proton is being converted to a neutron, it follows that the identity of the atom will change. More specifically, its atomic number will decrease by `1`.

Its mass number, which tells you the total number of protons and neutrons in the nucleus, will remain unchanged.

The nuclear equation for the electron capture decay of potassium-40 to argon-40 looks like this

`""_19^40"K" + ""_text(-1)^0"e"^(-) -> ""_18^40"Ar" + nu_e`

About one in nine potassium-40 atoms decays to argon-40, the rest decay to calcium-40 via beta minus decay.