--- marp: true theme: default html: true --- # How long does radioactivity last? Years 9 and 10 Chemistry · Inside the atom --- ## Today **We are learning to** each unstable nucleus decays at random, but a large sample halves in a fixed time: its half-life. After one half-life half is left, after two a quarter, after three an eighth. Half-lives range from minutes to billions of years, which decides what each isotope is used for. - I can define half-life. - I can work out how much of a sample is left after a number of half-lives. - I can explain why an isotope's half-life suits a use. --- ## Why it matters > Doctors give patients a radioactive drug in the morning that is almost gone by the next day. Yet some uranium in the rocks under you was made before the Earth formed, and about half of it is still there. --- 💬 Discussion ## Engage: predict **A sample has 80 iodine-131 nuclei. Half of any sample decays in 8 days. How many are left after 16 days?** - 20 - 40 - 60 - 0: two halves make a whole --- ✍️ Activity ## Explore it {{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fhalf-life%2Fstep-2&code=widget%3A+atom%0Amode%3A+halflife%0Aisotope%3A+iodine-131%0Anuclei%3A+100%0A&fresh=1}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/half-life/?full=step-2#step-2) --- ✅ Check ## What happened? * About 20. In the first 8 days half of the 80 go, leaving 40. In the next 8 days half of those 40 go, leaving 20. It halves each time; it never takes the same number twice. --- ✅ Check ## What half-life is **What is the half-life of a radioactive isotope?** - The time for half the nuclei in a sample to decay - The time for one nucleus to decay halfway - Half the time it takes the whole sample to decay * Answer: The time for half the nuclei in a sample to decay --- ✅ Check ## Three half-lives **A sample starts with 800 nuclei. How many are left after 3 half-lives?** * Answer: 100 --- ✅ Check ## Carbon-14 **Carbon-14 has a half-life of 5730 years. How many years until a quarter of a sample is left?** * Answer: 11460 --- ✅ Check ## One nucleus **You watch one carbon-14 nucleus. When will it decay?** - Within 5730 years for certain - In exactly 5730 years - No one can say: it could be in a second or in 50 000 years * Answer: No one can say: it could be in a second or in 50 000 years --- ✅ Check ## Nothing changes it **A sample is heated, crushed and dissolved in acid. What happens to its half-life?** - Nothing: half-life is the same whatever you do to the sample - It gets shorter - It gets longer * Answer: Nothing: half-life is the same whatever you do to the sample --- ✍️ Activity ## Elaborate: fluorine-18 in hospitals {{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fhalf-life%2Fstep-4&code=widget%3A+atom%0Amode%3A+halflife%0Aisotope%3A+fluorine-18%0Anuclei%3A+100%0A&fresh=1}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/half-life/?full=step-4#step-4) --- ✅ Check ## After 330 minutes **Fluorine-18 has a half-life of 110 minutes. What percentage of it is left 330 minutes after it is made?** * Answer: 12.5 --- ✅ Check ## Why 2 hours **Fluorine-18 is used in PET scans: it is made in the hospital, injected, and its radiation is detected outside the body. Why does a half-life of about 2 hours suit this?** - Any half-life would do: it makes no difference - So the patient stays radioactive for years, ready for the next scan - Long enough to make, inject and scan, short enough that little is left in the body by the next day * Answer: Long enough to make, inject and scan, short enough that little is left in the body by the next day --- ✍️ Activity ## Elaborate: dating the past {{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fhalf-life%2Fstep-5&code=widget%3A+atom%0Amode%3A+halflife%0Aisotope%3A+carbon-14%0Anuclei%3A+400%0A&fresh=1}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/half-life/?full=step-5#step-5) --- ✅ Check ## An old fireplace **Charcoal from an old fireplace has an eighth of the carbon-14 that living wood has. Carbon-14's half-life is 5730 years. About how old is it, in years?** * Answer: 17190 --- ✅ Check ## The limit **Radiocarbon dating works back to about 50 000 years. Why not further?** - After about 9 half-lives less than a five-hundredth of the carbon-14 is left, too little to measure reliably - Nothing older than 50 000 years contains carbon - Carbon-14 stops decaying after 50 000 years * Answer: After about 9 half-lives less than a five-hundredth of the carbon-14 is left, too little to measure reliably --- ✅ Check ## More than 60 000 years **Sites in northern Australia show First Peoples were there more than 60 000 years ago. Radiocarbon dates the younger layers; older layers are dated by other methods, such as luminescence (when sand grains were last in sunlight). Why does using more than one method matter?** - Radiocarbon is always wrong - It doesn't: one method is enough - Independent methods that agree make the dates much more trustworthy * Answer: Independent methods that agree make the dates much more trustworthy --- ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Each unstable nucleus decays at random: no one can say which will go next, or when. - A large sample decays in a predictable way: half of it goes in one **half-life**, half of what is left in the next, and so on (1/2, 1/4, 1/8 …). - Half-life is a property of the isotope; heat, pressure and chemical reactions don't change it. - Short half-lives suit medical imaging (fluorine-18, about 2 hours); long ones suit dating (carbon-14, 5730 years; uranium-238, 4.5 billion years). --- ✅ Check ## Exit ticket **Iodine-131 has a half-life of 8 days. A hospital receives 40 g. How much is left after 24 days?** * 5 g (40 → 20 → 10 → 5: three half-lives).