Atomic Structure

A Helium Atom { Neutron Electron Nucleus Proton Particle Mass Charge 1 +1 1 none 1/1840th -1

X Li H C A Z 7 1 12 3 1 6 Nucleon number Symbol of the element Proton

1 2 3 H H H 1 1 1

4 3 He He 2 2

C C 12 14 6 6 Now lets have a closer look at the nuclei of these isotopes.

C C N 12 14 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).

Half Life

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 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… 1 half life later… 1 half life later… 1 half life later… 8 4 8 2 8 1 8 …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.

Types of Radiation

Types of Radiation [Use all of the other side of the paper] Alpha Beta Gamma Description Electric Charge Relative Atomic Mass Penetrating Power Ionising Effect Effect of Magnetic / Electric Field Uses Dangers

What is radioactive decay? Teacher notes This activity, summarising the three types of radiation, could be used to introduce the topic of radiation, as a plenary exercise or as a revision exercise.

How do materials affect radiation? Teacher notes This virtual experiment illustrates the penetrating power of the three different types of radiation. It could be highlighted that the GM tube, when switched on, is not at zero. This is because it is reading a small amount of background radiation. It should also be made clear to the students that they should not put their hands in front of any known radiation source, and that the hand in the experiment is only for illustrative purposes.

How do magnetic fields effect radiation? Teacher notes This activity on the effect of a magnetic field on the three types of radiation could be used as a plenary or revision exercise.

Using the information on the following slides to fill in your table Movie

What is alpha (α) radiation? Description 2 neutrons, 2 protons Note:– An alpha particle is the same as a helium nucleus Electric charge +2 Relative atomic mass 4 Penetrating power Stopped by paper or a few centimetres of air Ionizing effect Strongly ionizing Effect of magnetic/ electric field Weakly deflected

What is beta (β) radiation? Description High energy electron Electric charge -1 Relative atomic mass 1/1860 Penetrating power Stopped by a few millimetres of aluminium Ionizing effect Weakly ionizing Effect of magnetic/ electric field Strongly deflected

Gamma () radiation Description High energy electromagnetic radiation Electric charge Relative atomic mass Penetrating power Stopped by several centimetres of lead or several metres of concrete Ionizing effect Very weakly ionizing Effect of magnetic/ electric field Not deflected

Types of radiation and penetrating power Teacher notes This matching activity could be used as a plenary exercise on the penetrating power of radiation. Students could be asked to complete the activity in their books or on mini-whiteboards. The activity could be concluded by completion on the IWB.

Types of radiation and range in air Teacher notes This matching activity could be used as a plenary exercise on the range in air of the three types of radiation. Students could be asked to complete the activity in their books or on mini-whiteboards. The activity could be concluded by completion on the IWB.

Uses

What is radiation used for? Teacher notes This true-or-false activity could be used as a plenary or revision exercise on everyday uses of radiation, or at the start of the lesson to gauge students’ existing knowledge of the subject matter. Coloured traffic light cards (red = false, yellow = don’t know, green = true) could be used to make this a whole-class exercise.

How can radiation detect a fire? 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. α α smoke particle Teacher notes How a smoke alarm works: The ionization chamber made of two metal plates a small distance apart. One of the plates carries a positive charge, the other a negative charge. Between the two plates, air molecules are ionized by the alpha particles from the radioactive material. The result is positively charged oxygen and nitrogen ions in the air. The free electrons are negatively charged. The positive ions flow toward the negative plate, as the negative electrons flow toward the positive plate. The movement of the electrons registers as a small but steady flow of current. When smoke enters the ionization chamber, the current is disrupted as the smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the flow of electricity between the two plates in the ionization chamber. When the electric current drops below a certain threshold, the alarm is triggered. This means that less current is flowing through the air, which causes the alarm to sound.

How is radiation used in making paper? Teacher notes This five-stage animation sequence shows how beta radiation is used commercially to help control paper thickness in a paper mill.

How can radiation find leaks in pipes? Teacher notes This four-stage animation sequence shows how beta radiation can also be used to detect the location of cracks in a leaking pipe without having to dig up the whole pipe.

How can radiation detect cracks? Gamma rays can also be used to detect cracks after an object has been welded. Gamma rays are like X-rays. welded metal pipe welding flaws 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. Photo credit: © PAUL RAPSON / SCIENCE PHOTO LIBRARY Teacher notes Image shows radiographic testing of a pipe. The technician is using an X-ray machine to check the pipe for flaws, such as poor welding. The machine (green) sends X-rays through the pipe to radiographic film beneath it (black, taped to the bottom of the pipe). The X-rays are absorbed depending on the thickness and density of the metal, revealing areas that are too thin to be considered safe. As gamma rays are like x-rays, the technique would be exactly the same if the technician was using a gamma source instead of x-rays. photographic film

High Level nuclear waste Microbes can be killed using gamma radiation

Gamma rays can be used to treat brain tumours Increasing dose tumour healthy brain tissue view through the head Gamma rays can be used to treat brain tumours skull

Uses of radiation – activity Teacher notes This drag and drop activity could be used as a plenary exercise to check students’ knowledge of everyday uses of radiation. Class voting or the use of coloured traffic light cards could make this a whole-class exercise.

Dangers

Dangers of ionizing radiations Teacher notes This six-stage animation sequence shows how radiation has an effect on living tissue, and why it can be so dangerous. 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? Photo credit: © 2006 Jupiterimages Corporation

Calculating background radiation Teacher notes This twelve-stage interactive animation explains more about the sources of background radiation, and how to calculate your annual dose of radiation. At each stage of the animation, students should keep a running total of each radiation source that affects them, which will give them a final figure at the end of the sequence. At stage 5: the easiest way to calculate the amount of TV watched per year or time spent on the computer, is to calculate the number of hours spent watching TV or at a computer per day, and then multiply this by 365, which will give an average annual amount.