Astatine is a radioactivechemical element wi the chemical seembolAt an atomic nummer 85. It occurs on Yird anly as the result o the radioactive decay o certain hivier elements. Aw o its isotopes are short-lived; the maist stable is astatine-210, wi a hauf-life o 8.5 oors. Accordinly, much less is kent aboot astatine nor maist ither elements. The observed properties are consistent wi it bein a hivier analog o iodine; mony ither properties hae been estimatit based on this resemblance.