Francium is radioactive because it contains unstable nuclei and is able to naturally release energy in the process of shedding high speed charged particles, in an attempt to reach a stable state.

Caesium is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C, which makes it one of only five elemental metals that are liquid at or near room temperature.The most common radioactive form of cesium is Cs-137.

Francium is a chemical element with the symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium K after the natural decay chain in which it appears), has a half-life of only 22 minutes.

Lithium is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Lithium is not radioactive.

Potassium is the chemical element with the symbol K and atomic number 19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to form flaky white potassium peroxide in only seconds of exposure. Potassium (K) is a widely encountered element with a very small fraction of its atoms, about 0.012%, being radioactive.