Potassium is represented by the letter K which stands for kalium. How we got about calling this stuff potassium is a sordid history which you can glance at here. Potassium has 3 naturally occurring isotopes (the percent abundance is in parenthesis): 39K (93.3%), 40K (0.0117%) and 41K (6.7%). So you can see that the majority of potassium is stable, with 19 protons and 20 neutrons. Just a wee bit of all the potassium that exists on this planet is radioactive Potassium, which has 20 protons and 20 neutrons.
Potassium-40 is a radioactive isotope that decays sometimes to Calcium-40 and other times to Argon-40. How can an isotope have two different daughters? Before we take this question to Maurie Povitch, let’s take a look at a few ways a radio-isotope can decay.
(REFRESHER!Protons have a positive charge and live in the nucleus of an atom; neutrons have no charge and also live in the nucleus of an atom; and electrons have a negative charge and whizz around outside the nucleus of an atom )
Alpha decay: This is when the isotope shoots off an alpha particle. An alpha particle is basically a Helium atom without any electrons (i.e.: 2 protons and 2 neutrons). As long as we’re on the subject of alpha decay, I should note that alpha particles have a short mean free path, meaning that they loose a lot of their energy a short distance after they are ejected from their mother-atom. So if an alpha particle hits your skin, it’s not going to do much damage. But if you eat or inhale alpha particles, you’re in trouble. They’ll get inside your stomach and lungs and cause all sorts of problems. That’s why you don’t want to eat or drink anything while you’re workin’ with alpha emitters, so take those tacos outside the lab!
Alpha Decay, with pizazz.
Beta minus decay: Also called “electron capture” because this is what happens when a proton turns into a neutron in the nucleus of your atom. The nucleus acts like a tractor beam and sucks in an electron which negates the charge on one proton.
Beta plus decay: With this decay method, the nucleus kick out what basically amounts to an electron, thereby turning a neutron into a proton.
There are a couple more decay pathways that I won’t get into. Anywho, back to potassium. Potassium-40 decays about 11% of the time by electron capture to Argon-40, and it decays about 89% of the time by beta plus to Calcium-40. We like it when potassium decays to argon because we can use these two isototpes to age date igneous rocks! When igneous rocks solidify, they’ll trap any potassium and any subsequent argon that is created by radioactive decay. Argon is a gas, but it’ll hang out inside the crystal lattice of a mineral just so long as that mineral isn’t heated. It gets to be tricky business if your rock/mineral *has* been heated, but don't worry your pretty little heads about it. Geochronomitry is in good hands.
Wow! Radioactivity, explosions, impotence…potassium really is pretty awesome!