Radon-219 decay chain

The ²¹⁹Rn isotope is a decay product of ²³⁵U. The half life of ²³⁵U is around 700 million year, which is less than a sixth of the age of our solar system. This means that only 1.1% of the amount of ²³⁵U that was present at the time that our solar system was created is left.

Radon-219 decay chain