eli5: How do radioactive elements form? How do they decay into other elements? If an element (such as Francium) only exists in miniscule amounts at any one time, how does it form naturally?

Question

eli5: How do radioactive elements form? How do they decay into other elements? If an element (such as Francium) only exists in miniscule amounts at any one time, how does it form naturally?

In: Chemistry

Answer 1

Elements are made in stars, which begin by fusing hydrogen together into helium

When they start to run out of hydrogen they fuse heavier elements, but this caps at iron because beyond that elements take more energy to fuse together than is released.

Elements heavier than iron are rarer because they are made by more extreme and rarer processes like when a star explodes

Elements decay because the nucleus is not stable from having an imbalance of protons and neutrons. Some elements have a stable form that doesn’t decay, some don’t.

For Francium there is about 1g in the earth at any time, but it is continually produced because it is part of the decay chain for Uranium 235, and U-235 is pretty long lived so there is still plenty of it around

Specifically:

U-235 has a half-life of 700 million years

It gives off an alpha particle to become Thorium 231, which has a half-life of a day before becoming Protactinium 231 by giving off a beta particle

Pa231 has a half-life of 30,000 years or so and emits an alpha particle to become Actinium 227, this has a half life of 22 years and usually decays to another form of Thorium (227) but slightly more than 1% of the time becomes Francium 223

That in turn has a half-life of 22 minutes but as long as there is U-235 around it will continue to be made in tiny amounts