Radioactive elements tend to be located primarily at the bottom of the periodic table, especially in the actinide series, which is a row of heavy elements known for their radioactivity. These elements include uranium, thorium, and others used in nuclear fuel cycles

. More specifically:

• The actinides, found in the f-block at the bottom of the table (atomic numbers 89 to 103), are mostly radioactive

• Elements with atomic numbers greater than 84 are generally radioactive due to their unstable nuclei, which have an imbalance of protons and neutrons causing radioactive decay

• Some radioactive elements also appear in other parts of the table, such as technetium (atomic number 43), which is lighter but has no stable isotopes and is entirely radioactive

In summary, radioactive elements are mostly found in the bottom rows of the periodic table (actinides and some transactinides), but radioactive isotopes can occur throughout the table. The heaviest elements tend to be radioactive because their large nuclei are unstable