The actinides and lanthanides make up the f-block of the periodic table. These two rows are frequently separated from the main table, but that’s just so the periodic table isn’t so wide on a piece of paper (seriously). Many of these elements have radioactive isotopes, but the rate at which they decay can vary immensely. For example, ²³⁸U has a half-life of 4.5 billion years, but ²³⁴Pa has a half-life of only seventy-two seconds!