2 Half - lifeThe time required for half ½ the nuclei in a sample of a radioactive isotope to decayThe half life for any radioactive isotope is a constant for any radioactive isotope.Each radioisotope decays at a unique rate
3 Time = 0100% sample (8 pennies) are still radioactive
4 Time = 1 half life50% of sample (4 pennies) are still radioactive
5 Time = 2 half lives25% of sample (2 pennies) are still radioactive
6 Time = 3 half lives12.5% of sample (1 penny) still radioactive
7 Time = 4 half lives No more pennies are radioactive. Think about a sample with millions of atoms…
8 7.2 Half-lifeIt can be difficult to determine the ages of objects by sight alone.Radioactivity provides a method to determine age.Measure the relative quantities ofremaining radioactive materialstable products formed.See pages(c) McGraw Hill Ryerson 2007
9 7.2 Half-lifeCarbon dating measures the ratio of carbon-12 and carbon-14.Stable carbon-12 and radioactive carbon-14 exist naturally in a constant ratio.When an organism dies, carbon-14 stops being created and slowly decays.Carbon dating only works for organisms less than years old..See pages(c) McGraw Hill Ryerson 2007
10 Carbon Dating – carbon-14 Living things replace the carbon-12 and carbon-14 in their bodies when they are alive.However when living things die, they do not replace the carbon-14 as carbon-14 decays into nitrogen-14Carbon-14 takes 5715 years to decay half of its nuclei into nitrogen- 14Carbon-12 remains unchanged.Therefore something that has been alive within the past years contains enough nuclei of remaining carbon-14 to measure.The ratio of carbon-12 and carbon-14 can estimate the age of the sample
12 Carbon datingThis picture shows a skeleton and a model for C-14 decay. The arrows represent the amount of C-14 giving off it's radiation as time passes. Notice the amount goes down by half for every half life.
14 Decay CurveAll decay curves for any radioactive element look the same except for the length of the half life.
15 The Rate of Radioactive Decay Half-life measures the rate of radioactive decay.Half-life = time required for half of the radioactive sample to decay.Strontium-90 has a half-life of 29 years. If you have 10 g of strontium-90 today, there will be 5.0 g remaining in 29 years.See pages(c) McGraw Hill Ryerson 2007
16 The Rate of Radioactive Decay Decay curves show the rate of decay for radioactive elements.The curve shows the relationship between half- life and percentage of the original substance remaining.The Rate of Radioactive DecayThe decay curve for strontium-90
17 Common Isotope PairsParent isotope = the original, radioactive materialDaughter isotope = the stable product of the radioactive decayThe rate of decay remains constant, but some elements require one step to decay while others decay over many steps before reaching a stable daughter isotope.See page 307(c) McGraw Hill Ryerson 2007
18 Common Isotope PairsThere are many radioisotopes that can be used for dating.Carbon-14 decays into nitrogen-14 in one step.Uranium-235 decays into lead-207 in 15 steps.Thorium-235 decays into lead-208 in 10 steps.See page 307(c) McGraw Hill Ryerson 2007
19 The Potassium-40 ClockRadioisotopes with very long half-lives can help determine the age of very old things.The potassium-40/argon-40 clock has ahalf-life of 1.3 billion years.Argon-40 produced by the decay of potassium-40 becomes trapped in rock.Ratio of potassium-40 : argon-40shows age of rock.See pages