Atomic Structure and Radioactivity Models of the atom know: Plum pudding model of the atom and Rutherford and Marsden s alpha experiments, being able to explain why the evidence from the scattering experiment led to a change in the atomic model. Be able to explain the difference between the plum pudding and nuclear model of the atom. Niels Bohr s adaptation of the Rutherford model That Chadwick s experiments showed the existence of neutrons in the nucleus. Atoms are very small, having a radius of about 0-0 metres with a positively charged nucleus surrounded by negatively charged electrons. The radius of a nucleus is less than /0 000 (about x0-4 m).of the radius of an atom. That most of the mass of an atom is concentrated in the nucleus. The electrons are arranged at different distances from the nucleus (different energy levels). The electron arrangements may change with the absorption of electromagnetic radiation (move further from the nucleus; a higher energy level) or by the emission of electromagnetic radiation (move closer to the nucleus; a lower energy level). The electrical charges and relative masses of protons, neutrons and electrons. How to calculate the number of protons, electrons and neutrons in an atom or ion given the atomic number and mass number What an isotope is an atom with different numbers of neutrons but the same number of protons. Atoms turn into positive ions if they lose one or more outer electron(s). Radioactive decay and nuclear radiation Radioactive decay is a random process where an unstable nuclei becomes more stable. Activity is the rate at which a source of unstable nuclei decays and is measured in Becquerel (Bq) Count-rate is the number of decays recorded per second by a detector/geiger-muller tube. The nuclear radiation emitted may be a) an alpha particle (α) this consists of two neutrons and two protons which is the same as a helium nucleus, b) a beta particle (β) a high speed electron ejected from the nucleus as a neutron turns into a proton, c) a gamma ray (γ) electromagnetic radiation from the nucleus or d) a neutron (n). Know the penetration of alpha particles, beta particles and gamma rays through materials, their range in air and ionising power. Be able to apply your knowledge to the uses of radiation and evaluate the best sources of radiation to use in a given situation Nuclear Equations To be able to use the names and symbols of common nuclei and particles to write balanced nuclear equations that show single alpha (α) and beta (β) decay. This is limited to balancing the atomic numbers and mass numbers. Nuclear equations are used to represent radioactive decay. For example: Page of 8
Atomic Structure and Radioactivity continued Radioactive Decay know: That Radioactive decay is random and that the half-life of a radioactive isotope is 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 to fall to half its initial level. How to determine the half-life of a radioactive isotope from given information. How to calculate the net decline, expressed as a ratio, in a radioactive emission after a given number of half-lives. That radioactive isotopes have a very wide range of half-life values. The hazards associated with a radioactive material depends on the half-life. Radioactive Contamination - know That radioactive contamination is the unwanted presence of radioactive atoms with the hazard being the due to the decay of the contaminating atoms. The type of radiation emitted affects the level of hazard. Irradiation is when an object is exposed to radiation, but does not become radioactive. How to compare the hazards associated with contamination and irradiation. The precautions to protect against hazards from radioactive sources. Recognise the importance for the findings of studies into the effects of radiation on humans to be published and shared so that the findings can be checked by peer review. Background radiation - know Background radiation is around us all of the time. It comes from natural sources such as rocks and cosmic rays from space, man-made sources such as the fallout from nuclear weapons testing and nuclear accidents. The level of background radiation and radiation dose may be affected by occupation and/or location. Radiation dose is measured in sieverts (Sv) - 000 millisieverts (msv) = sievert (Sv) Uses of radiation Know that nuclear radiations are used in medicine for the exploration of internal organs and control or destruction of unwanted tissue. Be able to describe and evaluate the uses of nuclear radiations for exploration of internal organs, and for control or destruction of unwanted tissue Be able to evaluate the perceived risks of using nuclear radiations when given data and consequences Nuclear Fission know that: Nuclear fission is the splitting of a large unstable nucleus (e.g. uranium or plutonium). Spontaneous fission is rare, for it to occur the unstable nucleus a neutron is absorbed. The nucleus undergoing fission splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons plus gamma rays. Energy is released by the fission reaction. All of the fission products have kinetic energy and the neutrons may go on to start a chain reaction where the reaction is controlled in a reactor with controlled energy Page 2 of 8
release. The explosion of a nuclear weapon is caused by an uncontrolled chain reaction. How to draw/interpret diagrams representing nuclear fission/ chain reaction. Nuclear Fusion know that: Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation. Look at the specimen points above and make sure you understand them all and can recall the equations and how to draw the different diagrams correctly. Create revision notes to help you learn the content of the topic Answer the test questions on GCSE bitesize pages linked below Try some of the questions at the end of this document Use the Mark Schemes at the end of the document to help you learn / mark your questions. Use the resources listed below to supplement your revision: Freesciencelessons.co.uk http://freesciencelessons.co.uk/ o Atomic Structure http://freesciencelessons.co.uk/atomic-structure-3/ o Atomic Mass and Number http://freesciencelessons.co.uk/atomic-and-mass-number/ o Alpha Scattering Experiment http://freesciencelessons.co.uk/alpha-scattering-experiment-2/ o Radioactivity http://freesciencelessons.co.uk/radioactivity/ o Properties of Alpha, Beta and Gamma Radiation http://freesciencelessons.co.uk/properties-of-alphabeta-and-gamma-radiation/ o Nuclear Equations http://freesciencelessons.co.uk/nuclear-equations/ o Half life http://freesciencelessons.co.uk/half-life/ o Irradiation and Contamination http://freesciencelessons.co.uk/irradiation-and-contamination/ o Background Radiation http://freesciencelessons.co.uk/background-radiation/ o Nuclear Radiation in medicine http://freesciencelessons.co.uk/nuclear-radiation-in-medicine/ o Nuclear Fission and Nuclear Fusion http://freesciencelessons.co.uk/nuclear-fission-and-nuclear-fusion/ Atomic Structure GCSE Bitesize https://www.bbc.com/bitesize/topics/zqtmw6f Radioactivity, Scool webstie https://www.s-cool.co.uk/gcse/physics/radioactivity Page 3 of 8
Atomic Structure Past Paper Questions Q. The diagram shows a lithium atom. What is the mass number of this lithium atom? Tick one box. 3 4 7 0 () What is the atomic number of a lithium atom? Tick one box. 3 4 7 0 Give a reason for your answer. (2) (c) Complete the sentence. Choose the answer from the box. circles levels rings Page 4 of 8
The electrons in an atom orbit in different energy. () (d) Some atomic nuclei are unstable and decay by emitting an alpha particle or a beta particle. Complete the symbols for an alpha particle and a beta particle. Use answers from the box. 0 2 4 (3) (e) Doctors may use nuclear radiation to diagnose certain types of illness. The table below gives data about three radiation sources used. Each source emits beta radiation. Radiation source Half-life in minutes Carbon- 20 Nitrogen-3 0 Oxygen-5 2 Explain why oxygen-5 is likely to pose the least risk to a patient. (2) (Total 9 marks) Page 5 of 8
Q2. The diagram represents a helium atom. (i) Which part of the atom, K, L, M or N, is an electron? Part () (ii) Which part of the atom, K, L, M or N, is the same as an alpha particle? Part () A radioactive source emits alpha particles. What might this source be used for? Put a tick ( ) in the box next to your answer. to monitor the thickness of aluminium foil as it is made in a factory to make a smoke detector work to inject into a person as a medical tracer () (c) The graph shows how the count rate from a source of alpha radiation changes with time. Page 6 of 8
What is the count rate after 4 hours? counts per second () (Total 4 marks) Q3. In the early 20th century, scientists developed an alpha particle scattering experiment using gold foil. The diagram shows the paths of some of the alpha particles in the alpha particle scattering experiment. Explain how the paths of the alpha particles were used to develop the nuclear model of the atom. Page 7 of 8
(4) Niels Bohr adapted the nuclear model by suggesting electrons orbited the nucleus at specific distances. Explain how the distance at which an electron orbits the nucleus may be changed. (3) (Total 7 marks) Q4. A student models the random nature of radioactive decay using 00 dice. He rolls the dice and removes any that land with the number 6 facing upwards. He rolls the remaining dice again. The student repeats this process a number of times. The table below shows his results. Roll number Number of dice remaining 0 00 84 2 70 3 59 4 46 5 40 6 32 7 27 Page 8 of 8
8 23 Give two reasons why this is a good model for the random nature of radioactive decay.. 2. (2) The student s results are shown in Figure. Figure Use Figure to determine the half-life for these dice using this model. Show on Figure how you work out your answer. Half-life = rolls (2) (c) A teacher uses a protactinium (Pa) generator to produce a sample of radioactive material that has a half-life of 70 seconds. In the first stage in the protactinium generator, uranium (U) decays into thorium (Th) and alpha (α) radiation is emitted. Page 9 of 8
The decay can be represented by the equation shown in Figure 2. Figure 2 Determine the atomic number of thorium (Th) 234. Atomic number = () (d) When protactinium decays, a new element is formed and radiation is emitted. The decay can be represented by the equation shown in Figure 3. Figure 3 When protactinium decays, a new element, X, is formed. Use information from Figure 2 and Figure 3 to determine the name of element X. () (e) Determine the type of radiation emitted as protactinium decays into a new element. Give a reason for your answer. (2) (f) The teacher wears polythene gloves as a safety precaution when handling radioactive materials. The polythene gloves do not stop the teacher s hands from being irradiated. Explain why the teacher wears polythene gloves. (2) (Total 0 marks) Page 0 of 8
Q5. Nuclear fission and nuclear fusion are two processes that release energy. (i) Use the correct answer from the box to complete each sentence. Geiger counter nuclear reactor star Nuclear fission takes place within a. Nuclear fusion takes place within a. (2) (ii) State one way in which the process of nuclear fusion differs from the process of nuclear fission. () The following nuclear equation represents the fission of uranium-235 (U-235). Chemical symbols: Ba - barium Kr - krypton (i) Use the information in the equation to describe the process of nuclear fission. (4) (ii) An isotope of barium is Ba-39. Ba-39 decays by beta decay to lanthanum-39 (La-39). Complete the nuclear equation that represents the decay of Ba-39 to La-39. Page of 8
(3) (Total 0 marks) Q6. The names of three types of nuclear radiation are given in List A. Some properties of these three types of radiation are given in List B. Draw a straight line to link each type of radiation in List A to its correct property in List B. Draw only three lines. List A Type of nuclear radiation List B Property of radiation (3) Nuclear radiation is given out from the centre of some types of atom. What name is given to the centre of an atom? () (c) One of the substances in the table is used as a radioactive tracer. A hospital patient breathes in air containing the tracer. The radiation given out is measured by a doctor using a detector outside the patient s body. Substance Radiation given out Solid, liquid or gas X alpha gas Y gamma gas Z gamma solid Page 2 of 8
Which one of the substances, X, Y or Z, should be used as the tracer? Give two reasons for your answer.. 2. (3) (d) Radiation can also be used to kill the bacteria on fresh food. Give one reason why farmers, shop owners or consumers may want food to be treated with radiation. () (Total 8 marks) Q7. The equation below shows the process by which two atomic nuclei join to form a different nucleus. Where does the process shown by the equation above happen naturally? Tick ( ) one box. Inside the Earth Inside a nuclear power station Inside the Sun () Use the correct answer from the box to complete the sentence. fission force fusion The process of joining two atomic nuclei to form a different nucleus is called Page 3 of 8
nuclear. () (c) What is released during this process? Draw a ring around the correct answer. charge energy force () (Total 3 marks) Page 4 of 8
Mark schemes Q. 7 3 number of protons reason only scores if 3 chosen (c) levels (d) correct order only (e) shorter half-life (than the other sources) exposure time to radiation is shorter [9] Q2. (i) L (ii) M To make a smoke detector work. (c) 40 no tolerance [4] Q3. most alpha particles pass straight through the atom which shows that the atom is mostly empty space very few alpha particles are deflected through a large angle Page 5 of 8
which shows the atom contains a nucleus where the mass / charge of the atom is concentrated electron may absorb electromagnetic radiation full credit may be scored for a description of an electron emitting electromagnetic radiation (and) move further from the nucleus to a higher energy level [7] Q4. cannot predict which dice / atom will decay accept answers given in terms of roll a 6 cannot predict when a dice / atom will decay 3.6 to 3.7 (rolls) allow mark for attempt to read graph when number of dice = 50 2 (c) 90 (d) (e) (f) uranium beta proton number has gone up (as neutron decays to proton and e ) prevents contamination or prevents transfer of radioactive material to teacher s hands which would cause damage / irradiation over a longer time period. [0] Q5. (i) nuclear reactor star Page 6 of 8
(ii) nuclei are joined (not split) accept converse in reference to nuclear fission do not accept atoms are joined (i) any four from: (ii) neutron (neutron) absorbed by U (nucleus) ignore atom do not accept reacts do not accept added to forms a larger nucleus (this larger nucleus is) unstable (larger nucleus) splits into two (smaller) nuclei / into Ba and Kr releasing three neutrons and energy accept fast-moving for energy 56 (Ba) 57 (La) if proton number of Ba is incorrect allow mark if that of La is greater 4 accept e for β scores 3 marks [0] Q6. 3 lines correctly drawn mark for each correct line if more than one line is drawn from a box in List A all lines from that box are wrong 3 nucleus Page 7 of 8
accept nuclei do not accept nuclear (c) Y do not accept gamma any two from: do not accept other properties of gamma least dangerous (inside the body) do not accept not dangerous accept not as harmful as alpha (inside the body) least ionising penetrates through the body do not accept can be detected externally is a gas / can be breathed in accept it is not a solid (cannot score if Z chosen) if X chosen can score this gas mark if Z chosen can score both gamma marks 2 (d) any one from: do not accept kills bacteria longer shelf life accept stays fresh longer / stops it going bad / mouldy food can be supplied from around the world wider market for farmers cost to consumers (may be) lower less likely to / will not get food poisoning accept infection / disease / ill for food poisoning [8] Q7. inside the Sun fusion (c) energy [3] Page 8 of 8