ATOMIC PHYSICS. igcse 0625 physics notes

PPTs notes of IGCSE Notes of Physics 0625

1. ATOMIC PHYSICS The Nuclear Atom and Radioactivity The Nuclear Atom In 1911 Geiger and Marsden performed series of experiments under the direction of Rutherford which led to the planetary or nuclear model of atom.

2. Rutherford’s experiment on the scattering of alpha particles The radioactive material kept inside a thick lead box emits alpha particles. Beam of alpha particles are then allowed to fall on a thin gold foil in a vacuum place.

3. While passing through the gold foil, the alpha particles are scattered through different angles, but few were repelled so strongly that they bounced back.

4. Rutherford concluded that the atom must be largely empty space, with the positive charge and most of its mass concentrated in a tiny nucleus at the center. In his model much lighter electrons orbited the nucleus. Atom Matter is made up of very small particles called atoms Each atom has a very small and very dense core called nucleus. Most of the mass of atom is contained in the nucleus The electrons move in orbits around the nucleus. There are a lot of empty spaces within atom

5. A nucleus consists of a number of protons and neutrons. Protons and neutrons also known as nucleons. A proton has a unit positive charge.   A neutron is an uncharged particle of about the same mass as the proton. An atom is neutral because it contains an equal number of negatively charged electrons. So the net charge is zero. Helium p n n e e p Lithium e e e n n n p p n p

6. Nuclide A nuclide is an atom of a particular structure. Each element has nucleus with a specific number of protons. Nuclide notation A = nucleon number (mass number) Z = proton number (atomic number) X = chemical symbol of the element Example Proton number (atomic number) of carbon = 6, carbon nucleus has 6 protons. The nucleon number (mass number) of carbon is 12. So the number of neutrons in carbon nucleus is 12 – 6 = 6

7. Question : Describe the structure of lithium atom. The atom of lithium atom of nucleus consists of 3 protons and 4 neutrons. Around the nucleus 3 electrons are orbiting. Lithium e e e n n n p p n p

8. Proton number (atomic number) Proton number, Z, is defined as the number of protons in a nucleus. The number of electrons = the number of protons An element is identified by its proton number   Nucleon number (mass number) Nucleon number, A is defined as the total number of protons and neutrons in a nucleus.

9. Isotopes Isotopes are atoms with the same proton (atomic) number but different nucleon (mass) number. Example: Hydrogen deuterium tritium Isotopes of an element contain the same number of protons and the same number of electrons. So isotopes have the same chemical properties chemical reactions involve the electrons in an atom. However they have different physical properties because their mass is different. Some isotopes exist naturally. Isotopes can also be made artificially.

10. Radioactivity Radioactivity is the spontaneous and random disintegration (decay) of an unstable nucleus accompanied by the emission of energetic particles or photons . • The nuclei of some atoms are unstable. The nucleus of an unstable atom will decay to become more stable by emitting radiation in the form of a particle or electromagnetic radiation. • Random process means there is no way to tell which nucleus will decay, and cannot predict when it is going to decay. .  

11. • A spontaneous process means the process is not triggered by any external factors such as temperature of pressure There are three types of radiation that is alpha ( α ) , beta ( β ) and gamma ( γ ) .

12. Characteristic Alpha ( α ) Beta ( β ) Gamma ( γ ) Nature Helium nuclei or 2 p and 2 n Electrons, Electromagnetic radiation. Mass 4 1/2000 Charge +2e -e Neutral Speed Slow Fast Speed of light Ionizing ability High Medium Low Penetrating power Low Medium High Stopped by A few cm of air or a piece of paper A few mm of aluminium foil A few cm of lead Deflected by electric and magnetic fields Yes Yes No Characteristics of alpha beta and gamma

13. Ionizing effect • Radioactive emission has an ionizing effect • The 3 types of radiation are highly energetic and use their energy to remove electrons from the air molecules when they pass through air. • The ionization of an atom produces positive ion and negative ion (electron) • Due to their different charges and masses, they have different ionizing abilities.

14. Alpha Beta Gamma                         Dense – strong ionization power straight tracks - the alpha particle has a large mass and momentum so it is not easily deflected Very fast beta particles - thin, straight tracks. The slower beta particles - short, thick tracks which curve in random direction. The gamma rays do not produce clear or continuous tracks due to their low ionizing power

15. Penetrating power • The penetrating effect of alpha, beta and gamma radiation depends on their ionizing power. • Radiation which has a stronger ionizing power will have a lower penetrating effect. • The radiation emission loses some of its energy each time an ion pair is produced. • Alpha particles lose energy very quick as they move through a medium. After a short distance in the medium, the alpha particles would have lost almost all energy. So alpha particles have the lowest penetrating power.

16. Alpha particles can be stopped by paper, beta particles can penetrate through paper but can be stopped by thin metal ( aluminium ). Gamma rays can go through paper and metal sheet and can only be stopped by thick lead or concrete.

17. • Alpha and beta particles are deflected in an electric field because they are charged. The deflections are in opposite direction because they carry opposite charges. The deflection of beta is larger than alpha because mass of beta is less than mass of alpha.   • Gamma rays are not deflected because they do not carry any charge. Interaction with electric field

18. Interaction with magnetic field • Alpha particles and beta particles are also deflected when they pass through a magnetic field while gamma rays are unaffected.   • The direction of the deflection of alpha particles in the magnetic field can be found using Fleming’s left-hand rule.

19. Detectors for radioactive emissions Radioactive emissions can be detected with the help of Geiger-Muller tube (GM tube), gold leaf electroscope, cloud chamber and spark counter. Geiger-Muller tube (GM tube)

20. Cloud Chamber • It shows the path traveled by the ionizing radiation in air. • The radioactive produces ions in the air that is saturated with alcohol vapour . • The alcohol vapour condenses on the ions to make the tracks of the radiation visible. Alpha particles are best for this because it ionization power is high.

21. • The radioactive emission enters the tube through the mica window and ionizes the neon gas. • The electrons and positive ions are attracted towards the anode and cathode respectively. • When electrons are collected by the anode, a pulse of current is produces. • The pulses of current are counted by a scaler or ratemeter . • The scaler gives the number of counts over a certain period of time that is counts per minute / counts per second. • Initially the GM tube is switched on without the presence of any radioactive substance. The reading displayed by the ratemeter is known as the background count rates. • When the GM tube is used to detect radioactive emission, the background count rate is subtracted from the count rate obtained. Notes: Background radiation gives reading to the GM tube even though there is no radioactive source. Background radiation is always present due to natural radioactivity in the ground, bricks or buildings and cosmic radiation .

22. Spark counter • The spark counter consists of a wire gauze and a thin wire below it. • A high voltage is applied between the gauze and the wire. The voltage is adjusted until it is just below the value required to produce sparks. • When a radioactive source is brought near the wire gauze, the radiation ionizes the air below it. The motion of the ions to the gauze and the wire causes sparks to be produced.

23. The spark can be seen and heard. • Spark counters are suitable for alpha particles. Beta particles and gamma rays produce too few ions to produce sparks.

24. Safety precautions in handling radioactive substances Alpha, beta and gamma radiation can all damage living cells. Alpha particles, due to their strong ability to ionise other particles, are particularly dangerous to human tissue. Gamma radiation is dangerous because of its high penetrating power. However cells have repair mechanisms that make ordinary levels of radiation relatively harmless. Radioactive substances must always be handled with the correct procedures to prevent harmful effects to people and the environment .

25. Safety precaution for handling radioactive materials include: Use forceps or tongs for handling radioactive sources – don’t hold them directly. Do not point radioactive sources at living tissues. Store radioactive materials in lead-lined containers – and lock containers away securely.

26. Wear laboratory coats, long pants, closed-toe footwear and gloves when entering radioactive place . Stronger radioactive sources should be handled with robotic control systems behind steel, concrete, lead or thick glass panels .

27. Check the surrounding area for radiation levels above the normal background levels. Background radiation The radioactive radiation present around the environment because of radioactive materials in the environment. Background radiation is always present due to natural radioactivity in the ground, bricks or buildings, rocks and cosmic radiation (radiation comes from stars and sun).

28. Radioactive decay Radioactive decay is a self disintegrating process which an unstable nucleus emits nuclear radiation like alpha beta or gamma so as to become stable. • When a radioactive nucleus decays, its nucleus breaks up, emits an alpha particle or beta particle and energy, and forms a new atom of a different element. • A parent nuclide X changes into a daughter nuclide Y.

29. Alpha decay When a nucleus emits alpha particle, the atomic number decreases by 2 units and it mass number decreases by 4 units . And high amount of energy released.

30. Beta decay When a nucleus emits a beta particle, the mass number does not change but the atomic number increases by 1 . and high amount of energy released.

31. Gamma decay Gamma emission does not change the structure of the nucleus, it just makes the nucleus more stable. Gamma rays are emitted at the same time together with either an alpha or beta particle. When a nucleus ejects an alpha or beta particle, there is often some excess energy produced which will be released as gamma rays.

32. Half –life The half-life T 1/2 of a radioactive substance is the time for half of the radioactive nuclei to decay. All radioactive substances decay with the same pattern, as shown in the graphs below.

33. The graph shows that amount of substance decrease rapidly at first and then more and more slowly. We cannot say when the last atom will decay. Different radioactive substances decay at different rates some much faster than others. Question 1: The count-rate from a radioactive source falls from 400 to 50 in 3.0 minutes. What is the half-life? Ans : 1minutes

34. Question 2: The half-life of a radioisotope is 2400 years. The activity of a sample is 720 counts / s. How long will it take for the activity to fall to 90 counts / s ? Ans : 7200 years Question 3: The half-life of a radioactive material is 24 years. The activity of a sample falls to a fraction of its initial value after 72 years. What is the fraction ? Ans : 1/8

35. Question 4: A radioactive isotope has a half-life of 6000 years. How much time passes before the rate of emission from a sample of this isotope falls to 1/16 of its original value ? Ans : 2400 years. Question 5: Figure shows how the number of atoms of a radioactive isotope changes with time. Determine the half-life of the radioactive isotope. Ans : 12 s

36. Nuclear fission Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei, when the nucleus of an atom is bombarded with a neutron. The energy of the neutron causes the target nucleus to split into two (or more) nuclei that are lighter than the parent nucleus, releasing a large amount of energy during the process.

37. When a neutron hits a uranium-235 nucleus, causes it to split producing strontium-90, xenon-143 and three neutrons + energy. Use of nuclear fission Electricity can be generated from the energy released by fission reactions. A nuclear power station consists of a nuclear reactor and a generator.

38. Nuclear energy of uranium Nuclear fission in nuclear reactor Heat energy turbine Heat energy of steam Mechanical energy generator Electrical energy Heat exchanger

39. Nuclear fusion Nuclear fusion is the combining of two lighter nuclei to form a heavier nucleus, releasing a vast amount of energy during the process.   Nuclear fusion is believed to be the process by which energy is released by the Sun. When two hydrogen-2 nuclei moving at high speeds collide, they can join together to produce heavier nucleus. A large amount of energy is released.

40. A hydrogen bomb uses the principle of nuclear fusion for its design .

41. Formation of star Distribution of hydrogen and interstellar dust in space may accidentally become so dense that they contract under their own gravity, causing temperature and density to rise.

42. When the mass starts to give a red glow, a protostar is formed. When temperatures at the center of the mass increase to ten-million degrees Kelvin, hydrogen will fuse to form helium in nuclear fusion reactions . Unlike ordinary chemical reactions we are familiar with, nuclear reactions convert one chemical element into another such as from hydrogen (H) to helium (He), releasing a lot of energy, which causes the temperature to rise further. The energy causes the surface to heat up, and eventually, energy escapes from the mass as radiation (heat and light). At some point in time, the state is steady in that the amount of energy released from fusion reactions equals to the amount lost by radiation, and we call such a collection of mass a star .

43. Carbon dating Radioactive substances decay at a rate we can determine, we can use them to discover how old objects and animals are. The best-known example of this is radiocarbon dating . All living things contain carbon. Plant get this from atmospheric carbon-dioxide, which they use in photosynthesis. Plant-eating animals get it from the plants they eat to build their bodies. Meat eating animals get it from their prey. Most carbon is carbon – 12 ( ), which is not radioactive. A tiny fraction is radioactive carbon – 14 ( ), with a half-life of 5370 years. (It emits beta radiation)

44. The idea of radiocarbon dating is this. When a living organism dies, the carbon – 14 in its body decays. As time passes, the amount remaining decreases. If we can measure the amount remaining, we can work out when the organism was alive. There are two ways to measure the amount of carbon – 14 present in an object. By measuring the activity of the sample using a detector such as a Geiger counter. By counting the number of carbon – 14 atoms using a mass spectrometer.