Radioactive Elements
Uranium-238 half life of 4.5 billion years
Uranium-235 half life of 0.7 billion years (but only 0.7% of Uranium is U-235)
Thorium-232 half life of 14 billion years
Potassium-40 1.3 billion years
(but only 1 out of 5000 Potassium atoms are K-40)
Neptunium-237 has a half life of 2.2 million years but is not found in nature.
Plutonium-244 has a half life of 76 million years but is not found in nature.