#radioactive element

• Actinides — Plural form for the neuter noun 'actinid'; a homogeneous structural group in the periodic table consisting of 15 metallic and radioactive chemical elements, with atomic numbers ranging from 89 (actinium) to 103 (lawrencium). Located in the 'f' block of transition elements in the 7th period, they exhibit similar chemical properties due to the progressive filling of the 5f electron subshell, being recognized for their high density, substantial nuclear instability, and capacity to undergo fission.
• At — The official chemical symbol for the element astatine, an extremely rare and unstable radioactive halogen with atomic number 85. In modern periodic tables, At represents the heaviest characterized element in the halogen group, whose physical and chemical properties are theoretically deduced or microscopically observed in high-energy nuclear physics laboratories due to its very short half-life.