High Quality Content by WIKIPEDIA articles! Radium (Ra) has no stable or nearly stable isotopes, and thus a standard atomic mass cannot be given. The longest lived, and most common, isotope of radium is 226Ra with a half-life of 1600 years. 226Ra occurs in the decay chain of 238U (often referred to as the radium series.) Radium has 35 known isotopes from 201Ra to 235Ra, with the newest being 235Ra.