Astatine

Astatine is a highly radioactive chemical element with atomic number 85 and symbol At. It occurs naturally only in trace amounts as a product of radioactive decay.

Halogen: A halogen is an element in group 17 of the periodic table, including fluorine, chlorine, bromine, iodine, and astatine. Astatine occupies the group's bottom position, where periodic trends become difficult to predict.
Alpha decay: Alpha decay is a nuclear process in which an unstable nucleus emits a helium-4 nucleus. Some radioactive decay chains produce astatine through alpha-particle emission.
Targeted alpha-particle therapy: Targeted alpha-particle therapy delivers alpha-emitting radionuclides to diseased cells while limiting radiation to surrounding tissue. Astatine-211's emissions and brief half-life suit localized treatment strategies.
Iodine: Iodine is a halogen element with atomic number 53 and symbol I. Iodine's well-characterized chemistry helps clarify how astatine differs from a lighter halogen.
Periodic table: The periodic table arranges chemical elements by atomic number and recurring chemical properties. Astatine's placement among the halogens frames expectations about its properties.
Beta decay: Beta decay is a nuclear transformation in which a neutron becomes a proton or a proton becomes a neutron, emitting other particles. Beta transitions in heavy-element decay chains can form astatine isotopes.
Alpha particle: An alpha particle is a helium-4 nucleus consisting of two protons and two neutrons. Astatine-211 releases alpha particles that deposit energy over short distances.
Polonium: Polonium is a highly radioactive chemical element with atomic number 84 and symbol Po. Unlike astatine, polonium belongs to group 16 rather than the halogens.
Atomic number: The atomic number is the number of protons in an atom's nucleus and defines its chemical element. Astatine's 85 protons distinguish it from every other element.
Astatine-210: Astatine-210 is a radioactive isotope of astatine with 125 neutrons. Its decay illustrates how an astatine isotope can transform into another element.

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