If we know the half life of an isotope, we can work out how much will be left after a certain amount of time. Example: The half life of H3 (Tritium) is 4,500 days. If we had 12 grams to begin with, how much would there be after 13,500 days? Number of half lives = 13,500 / 4,500 = 3 Mass left after 3 half lives = 12 g × 0·5 × 0·5× 0·5 = 1·5 g Notice how we half it three times... 12>6>3>1.5
