Stable and Unstable Elements

There are at the moment 115 known elements with atomic numbers 1-115. Of these, 93-115 exist only as the results of nudear reactions. Strictly speaking the natural elements are less numerous than 92 as some occur only as radioactive isotopes with such short half-hves that their concentrations in nature are extremely low. These special elements are 43 technetium, 61 promethium, 84 polonium, 85 astatine, 86 radon, 87francium, 88 radium and 89 actinium. 

These exist or existed as intermediate products in radioactive decay series. It does not mean that they are unimportant The gaseous element radon may, as is well known, be present as a dangerous air pollutant in houses built on radioactive ground. Because the element is gaseous the radioactivity can be removed by ventilation. 

Some radioactive elements have isotopes with very long half-lives, for instance 90 thorium and 92 uranium. The element 91 protaotinium. Pa, has an intermediate posi- 

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