every chemical element goes through natural exponential decay, which means that over time its atoms fall apart. the speed of each element's decay is described by its half-life, which is the amount of time it takes for the number of radioactive atoms of this element to be reduced by half. the half-life of the isotope beryllium-11 is 14 seconds. a sample of beryllium-11 was first measured to have 800 atoms. after t seconds, there were only 50 atoms of this isotope remaining.