From a book called The Age of the Earth by G. Brent Dalrymple . . .

1. Potassium (K) is common in rocks and there are several minerals in which it is the principle element. A .00001 g sample of K contains 150,00 trillion atoms, including over 17 trillion atoms of the radioactive K-40.

2. Argon (Ar) is an inert gas that does not combine with other elements.

3. While the rock is molten the Ar formed by the decay of K escapes the magma. After the rock has solidified and cooled, the argon is trapped within the crystal structure of the minerals like bird in a cage, accumulating with the passage of time.

4. K-40 (radioactive type, or isotope of potassium) has a nearly ideal half-life of 1.25 billion years.

5. There are 339 isotopes of 84 different natural elements, including 269 that area stable and 70 that are radioactive (not stable).

6. There are three kinds of potassium (3 isotopes). Of these 93.26 % are K-39, .0117 % are K-40, and 6.73 % are K-41.

Here’s how it works . . .
A small amount (.0117%) of all K atoms are the radioactive isotope known as K-40. As years go by, the K-40 atoms gradually undergo radioactive decay, changing into Ar-40 atoms. Scientists know that it takes 1.25 billion years for half of the K-40 to change into Ar-40. This length of time is called the “half-life,” and it is different for other radioactive isotopes. For example, carbon-14, which is found in living things (and dead things) has a half-life of only 5,730 years. If geologists find that an igneous rock has a K to Ar ratio of 1 to 1, they would conclude that the magma hardened 1.25 billion years ago. If the ratio is 1:3, then two half-lives have gone by . . . the rock is 2.5 billion years old.