Actinides are all radioactive elements.

Solution

Chemistry ma’am · 4 steps

Look at where the actinides sit: every one of them comes after bismuth, and nothing there is stable.

1. Who they are. The actinides are the 15 elements from actinium to lawrencium, Z = 89 to 103. They fill the 5f orbitals.
2. Heavy nuclei. No element with Z greater than 83 has a stable isotope. These nuclei hold so many protons that the repulsion between them makes the nucleus break down over time.
3. Natural is not stable. Thorium and uranium are found in large amounts in nature only because their main isotopes decay very slowly. Their half-lives are about 1.4 × 10¹⁰ and 4.5 × 10⁹ years. Most of the later actinides are made in nuclear reactors or particle accelerators and decay quickly.
4. So. Every actinide, from Ac to Lr, is radioactive. The statement is true.

The idea to keep. Past bismuth (Z = 83), no element has a stable isotope. So every actinide is radioactive, and that includes the ones you can dig out of the ground.

Answer from the NTA final answer key. Solution by PrepWiser.