KS4 Radioactive decay and Model of the atom

Radioactive waste Radioactive waste from nuclear power stations is an environmental concern. The problem is the waste stays radioactive for thousands of years. The current solutions are: Store it at the nuclear power station until is filled up. Dump it far out at sea. Store it deep underground in non-permeable rock.

Radiation questions What are the three types of radiation? Which type of radiation is the most penetrating? Why is radioactive waste not stored in permeable rock? Why should nuclear power stations not be situated in geologically active regions? Alpha, beta and gamma Gamma It could contaminate water that seeps through the rock. Earthquakes could cause radioactive spills.

What we used to think… It was believed that atoms were: Spheres of positive charge. With negative charges spread through it. This resembled a plum-pudding, so it was called the ‘Plum –pudding’ model. This was wrong! How did we discover current ideas about the structure of the atom?

Rutherford’s team: Ernest Rutherford and his team of scientists performed a famous experiment in Manchester: They fired some alpha particles at a piece of thin gold foil (only a few atoms thick): If the ‘Plum Pudding’ model of the atom was correct, the alpha particles should pass straight through and only be slightly deflected. This did not happen.

What Rutherford’s team observed…….. Most of the alpha particles went straight through the foil. Some alpha particles were deflected through large angles. 3. A very few alpha particles were reflected straight back.

Rutherford’s conclusions Observation Conclusion Most alpha particles went straight through the foil. A few were deflected through large angles. A very few were reflected straight back. Atoms are mostly space. The nucleus is very small compared to the size of the atom and it contains most of the mass and all the positive charge.

Task Pretend you are Ernest Rutherford and you have just completed your investigation. Write a letter to a fellow scientist describing your observations and findings. Include the impact you think it will have on current thinking. Dear Dr. Banner, I am just writing to……

Label the helium atom and fill in the table: eutron { E_____ lectron N_____ ucleus P_____ roton Particle Mass Charge Proton Neutron Electron 1 +1 1 none 1/1840th -1

X N Notation A Mass number: The number of protons plus the number of neutrons in a neutral atom. A X N Element symbol Atomic number: The number of protons (which is the same as number of electrons) in a neutral atom.

Name the elements shown and calculate the numbers of protons, neutrons and electrons for the elements: 12 C 6 75 As 33 127 I 53

Use a periodic table to fill in the table below: Element Protons Neutrons Electrons H He Li Be B C N O F Ne Na Mg Al Si

Calculate the number of protons, electrons and neutrons shown below - 12 C 6 13 C 6 14 C 6 What do we call isotopes that are unstable and emit radiation to become more stable? What do we call atoms of the same element with different numbers of neutrons? These are all the element carbon, what is the difference between them? They have different numbers of neutrons. Radioisotopes Isotopes

Radioactive Decay Why is it that there are different types of radiation? What is going on inside the nucleus? The three types of decay are……… Alpha Gamma Beta

Type of decay: What is emitted? Description of decay: Example of decay: Effect on A and Z: Alpha decay Alpha particle (helium nuclei) 2 neutrons and 2 protons are emitted from the nucleus. 238 234 4 U  Th +  + energy 92 90 2 A decreases by 4, Z decreases by 2 (A-4, Z-2)

Type of decay: What is emitted? Description of decay: Example of decay: Effect on A and Z: Beta decay High energy electron A neutron in the nucleus decays into a proton and a high energy electron which is emitted. 14 14 0 C  N +  + energy 6 7 -1 A stays the same, Z increases by 1 (A=, Z+1)

High energy electromagnetic radiation. Type of decay: What is emitted? Description of decay: Effect on A and Z: Gamma decay High energy electromagnetic radiation. Nucleus changes shape into a more stable shape. Gamma radiation emitted as a result. A stays the same, Z stays the same (A=, Z=)

You must learn both of these definitions! Half life There are two definitions of half life: The time it takes the number of radioactive nuclei in a sample to decrease by 50%. The time it takes the count rate from a radioisotope to decrease by 50%. You must learn both of these definitions!

Graphical representation of half life Decay rate (counts/min) What is the half life of the radioisotope represented by the following graph? 80 60 40 20 The time it takes the count rate to decrease from 80 per min to 40 per min is what? 2 mins Double check, the time it takes the count rate to decrease from 40 per min to 20 per min is? 2 mins 2 4 6 8 Time (min) The half life of the radioisotope is 2 mins.

Half life questions What are the two definitions of half life? If 1/64th of an original radioisotope is left after 1 hour, what is the half life of the sample? A radioisotope has a half life of 12 minutes. What fraction of the radioisotope will be left after 2 hours? The background radiation in a laboratory is 13 counts per minute. The count rate from a radioisotope is measured and it has a reading of 119 counts per minute. If the half life of the radioisotope is 10 minutes, what will be the reading 20 minutes later?

Carbon Dating All living things take in a little radioactive carbon-14 in photosynthesis, as well as the normal carbon-12. When living things die, they stop taking in carbon-14 and so the carbon-14 present at death slowly decays to carbon-12 (half-life is 5 600 years). The radioactivity due to the decay of carbon-14 can be used to date bones, wood, paper and cloth.

Example A fresh bone gives a radioactive count of 170 counts per minute. Another ancient bone of the same mass gives a count rate of 50 counts per minute. The background count is 10 counts per minute. How old is the bone? click Counts due to bones are 170 - 10 = 160 (fresh) and 50 - 10 =40 (ancient) The count rate of the carbon-14 has fallen to one quarter of its original value, i.e. 160/2 = 80, 80/2=40. This is two half lives, So the bone is 5600 x 2 =11200 years old.

Nuclear Power When a nucleus decays it gives out heat energy. In a nuclear power station, the uranium-235 atoms decay and give out energy and neutrons. Each time a uranium atom splits it produces 2 or 3 neutrons (depending on the reaction). These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled. In a nuclear bomb the reaction is not controlled, and the bomb explodes!

Nuclear Power - fission Fast neutron from previous decay cause the Uranium nucleus to split.

Fission Nuclear Power Kr n More decays n n Uranium n Ba In the reaction above a neutron from a previous decay can lead to more and more decays. Ba This is called a chain reaction.

During alpha decay, which of the following is true? Relative atomic mass increases by 2 Relative atomic mass decreases by 2 Relative atomic mass increases by 4 Relative atomic mass decreases by 4 

During beta decay, which of the following is true? Atomic number increases by 1 Atomic number decreases by 1 Atomic number increases by 2 Atomic number decreases by 2 

What fraction of a radioactive sample is left after 4 half lives? 1/2 1/4 1/8 1/16 