If we start with 400 atoms of a radioactive substance, how many would remain after one half-life? two half-lives? after three half-lives? 2. If we start with 48 atoms of a radioactive substance, how many would remain after one half life? two half-lives? 3. If we start with 16 grams of a radioactive substance, how much will remain after three halflives? 4. If we start with 120 atoms of a radioactive substance, how many will remain after three half-lives? Half-life 4 days 8 days 1600 years 5,730 years 24,120 years 4,470,000,000 5. If we start with 8000 atoms of radium-226, how much would remain after 3,200 years? 6. If we start with 20 atoms of plutonium-239, how many would remain after 48,240 years? 7. If we start with 60 atoms of uranium-238, how many remain after 4,470,000,000 years? 8. If we start with 24 atoms of iodine-131, how many remain after 32 days? 9. Sodium-24 has a half-life of 15 hours. How much sodium-24 will remain in an 18.0g sample after 60 hours? Radioactive substance Radon 222 lodine - 131 Radium-226 Carbon-14 Plutonium-239 Uranium-238 10. The half-life of radium-226 is 1600 years. How many grams of a 0.25g sample will remain after 4800 years? 11. The half-life of lodine-131 is 8 days. How many grams of a 284g sample will remain after 32 days? 12. After 42 days a 2.0 g sample of phosphorus-32 contains only 0.25g of isotope. What is the half-life of phosphorus-32? 13. After 20 days a 532.0 g sample of Radon-221 contains only 33.25g of isotope. What is the half-life of Raadon-221? 14. An 80g sample of Isotope Q has only 1.25g remaining after 30 hours what is the half-life of isotope Q's? 15. After 120 hours a 28 g sample of isotope-x contains only 1.75 g of isotope x. What is the half-life of isotopex? 16. The half-life of thorium-227 is 18.72 days. How many days are required for 75% of a given amount to decay? after after