Radium (88Ra) has no stable or nearly stable isotopes, and thus a standard atomic weight 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 33 known isotopes from 202Ra to 234Ra. In 2013 it was discovered that the nucleus of radium-224 is pear-shaped. This was the first discovery of an asymmetric nucleus.