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Atomic Structure. A Helium Atom. Neutron. {. Electron. Nucleus. Proton. 1. +1. none. 1. 1/1840th. -1. Nucleon number. A. X. Symbol of the element. Proton number. Z. 7. 1. 12. Li. H. C. 3. 1. 6. 1. 2. 3. H. H. H. 1. 1. 1. 4. 3. He. He. 2. 2. 12. 14.
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A Helium Atom Neutron { Electron Nucleus Proton 1 +1 none 1 1/1840th -1
Nucleon number A X Symbol of the element Proton number Z 7 1 12 Li H C 3 1 6
1 2 3 H H H 1 1 1
4 3 He He 2 2
12 14 C C 6 6 Now lets have a closer look at the nuclei of these isotopes.
12 14 C C N 6 6 7 Carbon-12 is stable but Carbon-14 is unstable (a radio-isotope). Carbon-14 emits a beta particle and decays to become nitrogen
Calculate the number of protons, electrons and neutrons shown below - 12 C 6 13 C 6 14 C 6
Notes from Syllabus: • An atom has a small central nucleus made from protons and neutrons surrounded by electrons. • All atoms in an element have the same number of protons. • An atom can have different isotopes (different number of neutrons).
How about with Real Atoms • Look at the generated graph. • How long does it take for ½ of the atoms to decay? • How long for 3/4? • How long for 7/8? • How long for 15/16 • Decay
Half Life • Half-life is the time it takes for half of the atoms of a sample to decay. • For example: • A student was testing a sample of 8 grams of radioactive protactinium. Protactinium has a a half life of 1 minute and decays into actinium. • After 1 minute there would be 4 g of protactinium (and 4 g of actinium). • After 2 minutes there would be 2 g of protactinium remaining (and now 6g of actinium). • After 3 minutes there would be 1 g of protactinium remaining (and now 7g of actinium)
Dating materials using half-lives 1 half life later… 1 half life later… 1 half life later… 2 1 8 4 8 8 8 8 Question: Uranium decays into lead. The half life of uranium is 4,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. Answer: The sample was originally completely uranium… …of the sample was uranium Now only 4/8 of the uranium remains – the other 4/8 is lead Now only 2/8 of uranium remains – the other 6/8 is lead Now only 1/8 of uranium remains – the other 7/8 is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000 years so the sample is 12,000,000 years old.
An exam question… Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years
Notes from Syllabus: • Radioactive substances emit radiation from the nuclei of their atoms all the time. • The half-life of a radioactive isotope is • Either the time it takes for the number of nuclei of the isotope in a sample to halve • or the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.
Using the information on the following slides to fill in your table Movie
What is alpha (α) radiation? 2 neutrons, 2 protons Description Note:– An alpha particle is the same as a helium nucleus +2 Electric charge 4 Relative atomic mass Stopped by paper or a few centimetres of air Penetrating power Strongly ionizing Ionizing effect Effect of magnetic/ electric field Weakly deflected
What is beta (β) radiation? High energy electron Description -1 Electric charge 1/1860 Relative atomic mass Stopped by a few millimetres of aluminium Penetrating power Weakly ionizing Ionizing effect Effect of magnetic/ electric field Strongly deflected
Gamma () radiation High energy electromagnetic radiation Description 0 Electric charge 0 Relative atomic mass Stopped by several centimetres of lead or several metres of concrete Penetrating power Very weakly ionizing Ionizing effect Effect of magnetic/ electric field Not deflected
How can radiation detect a fire? smoke particle α α Smoke alarms contain a weak source of alpha radiation. The alpha particles ionize the air. If there is smoke present, it interacts with the ions produced by the alpha particles and ionization is reduced. This means that less current is flowing through the air, which causes the alarm to sound.
How can radiation detect cracks? welded metal pipe welding flaws photographic film Gamma rays can also be used to detect cracks after an object has been welded. Gamma rays are like X-rays. If a gamma source is placed on one side of the welded metal, and a photographic film on the other side, any flaws will show up on the film like an X-ray.
High Level nuclear waste Microbes can be killed using gamma radiation
Increasing dose tumour healthy brain tissue view through the head Gamma rays can be used to treat brain tumours skull
Dangers of ionizing radiations Fill in the last part of your table
Radiation safety The three types of radiation differ in their effects and physical nature. All radioactive sources must be handled safely. The hazard symbol for radiation is shown below: As well as the normal laboratory safety rules you follow, are there any extra rules concerning radioactivity?
How are radioactive sources used safely? Radioactive materials could be very dangerous to handle if no safety precautions were taken. This is because people and their clothing could become contaminated. Write down on first side of the paper The safety precautions are: • keep exposure times as short as possible • monitor exposure with a film dose badge • label radioactive sources clearly • store radioactive sources in shielded containers • wear protective clothing • use tongs or a robotic arm to handle radioactive materials.
Background radiation Background radiation is the radiation all around us. Most of the radioactivity you are exposed to is from natural sources. How many different sources of background radiation can you think of?
