A question asked by a friend caused me to use Periodic Chart software to check on the properties of various elements. For every element, the abundance of each isotope was provided as a percentage. This data was included for both radioactive and stable isotopes. For example, the abundances for uranium were 0.0055% for U<sub>234</sub>, 0.72% for U<sub>235</sub>, and 99.27% for U<sub>238</sub>. There were a lot of other isotopes of Uranium listed with zero abundances. For several elements (EG: Radium and actinium), all the abundances were zero. What does zero percent abundance mean? Not enough of the element to make a measurement? No valid measurements ever made? The periodic chart software is in error? The two elements I noticed (Radium & actinium) have no stable isotopes and are very rare. Could zero percent abundance mean that all of the element has decayed, leaving the Earth void of said element?