Physics Topic 4-Atomic structure

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How did the idea about atomic structure begin?
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Democritus – believes all matter was made from atomos
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What happened next in 1804?
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Dalton agreed with Democritus matter was made up of atoms that couldn’t be broken up
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What happened 100years later?
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Jj Thomson discovered particles called electrons that could be removed fro atoms Thomson suggested atoms were spheres of positive barge with electrons in them = PLUM PUDDING MODEL
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What happened after in 1909
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Rutherford tried firing alpha particles at thin gold foil = ALPHA SCATTERING EXPERIMENT Some particles went through the sheet, some deflected, some deflected back the way they had come – against plum pudding
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What did the deflection result in?
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Scientists realised most mass of the atom was concentrated at the centre in a nucleus – nucleus has to be positively charged as it repelled alpha particles Also realised atom was just empty space as most particles passed straight through
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What was the result of the alpha scattering experiment?
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The nuclear model – positively charged nucleus surrounded by cloud of electrons
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What did Bohr say?
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Electrons orbiting nucleus do so at certain distances called energy levels
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What did further experiments find?
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Nucleus made up of group of protons which all had same positive charge
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What happened in 1932 when the idea of the nucleus was accepted?
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Chadwick proved existence of neutron which explained the imbalance between atomic and mass numbers
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What is the relative charge of a proton?
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+1
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What is the relative charge of a neutron?
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What is the relative barge of an electron?
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-1
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The number of protons =
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The number of electrons
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What is an isotope?
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Different form of the same element
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What is the atomic number?
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The number of protons
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What is the mass number?
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Number of protons and neutrons
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What is the difference with an isotope?
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Same atomic number different number of neutrons
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Where is the atomic number in an element?
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At the bottom – the smaller number
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Where is the mass number in an element?
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At the top – the larger number
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What happens to unstable isotopes?
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Decay into other elements Give out radiation to become more stable (Process called RADIOACTIVE DECAY)
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What are the three types of radiation emitted from the nuclei of radioactive substances?
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• Alpha • Beta • Gamma
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What is ionising radiation?
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Radiation that knocks electrons off atoms, creating positive ions
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What is an alpha particle?
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Helium nuclei 2 neutrons and 2 protons Don’t penetrate far – absorbed by paper Strongly ionising due to their size
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What are beta particles?
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High speed electrons Virtually no mass Moderately ionising Moderately penetrating – absorbed by sheet of aluminium When one is emitted a neutron in the nucleus turns into a proton
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What are gamma rays?
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EM waves with short wavelength released by nucleus Penetrate far into materials Weakly ionising as they pass through Do damage when hit something Absorbed by thick sheets of lead and metres of concrete
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When is alpha radiation used?
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In smoke detectors
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When is beta radiation used?
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Beta emitters are used to test thickness of sheets of metal
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What are gama
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What are nuclear equations?
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A way of showing radioactive decay using element symbols Atom before decay ➡️ atom after decay + radiation emitted
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What does the mass and atomic numbers have to be?
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Balanced Total mass and atomic numbers must be qual on both sides
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What does alpha decay decrease?
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The charge and mass of the nucleus When an atom emits alpha particle – atomic number reduces by 2 and mass number by 4 E.g. 238U –> 234Th + 4He 92. 90. 2
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What does beta decay increase?
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The charge of the nucleus The number of protons in a nucleus is increased by 1 E.g. 14C –> 14N + 0e 6. 7. -1
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What is different with gamma rays?
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They don’t change the charge or mass of the nucleus Get rid off excess energy from a nucleus No change to atomic mass or number
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What does activity and half life measure?
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How quickly unstable nuclei decay is
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How to measure radiation?
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Geiger-muller tube and counter which records the count rate
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What is count rate?
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Number of radiation counts reaching it per second
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What is half life?
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The tame taken for the number of radioactive nuclei in an isotope to halve The time it takes for the amount of radiation emitted by a source to halve Time taken for activity (count rate) to halve Used to find the rate at which a source decays
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What is activity measured in?
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Becquerels Bq (1Bq is 1 decay per second)
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What does radioactivity do overtime?
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Decreases
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What does a short half life mean?
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Activity falls quickly – nuclei unstable and rapidly decay
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What does a long half life mean?
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Activity falls slowly – nuclei don’t decay fit a long time
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What is background radiation?
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Low level radiation that’s surrounds us all the time
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Where does background radiation come from?
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Unstable isotopes (air, food, building materials, rocks) Space (cosmic rays) – sun Human activity – nuclear explosions/waste
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What is radiation dose?
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Tells you the risk of harm to body tissues due to exposure to radiation Measured in sieverts (Sv) Does is small so use millisieverts 1Sv = 1000 mSv
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What is Irradiation?
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Exposure to radiation
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How can we reduce (the effects of) irradiation?
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Keep sources in lead lines boxes Stand behind barriers Being in a different room Using remote controlled arms
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What is contamination?
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Radioactive particles getting into objects
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How can we reduce (the effects of) contamination?
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Use gloves and tongs and sometimes protective suits when handling sources
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What are the risks of using radiation?
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Can enter living cells and ionise atoms and molecules – tissue damage Lower doses = minor damage without killing cells –> give rise to mutant Cells which divide uncontrollably (CANCER) Higher doses – kill cells completely = radiation sickness = vomiting, tiredness, hair loss
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Where are gamma sources used?
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Medical tracers
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What is radiotherapy?
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Treating cancer with radiation
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What is nuclear fission?
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Splitting a large unstable nucleus Nuclear reaction used to release energy from large unstable atoms by splitting them into smaller ones
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Why is nuclear fission spontaneous?
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Rarely happen as the nucleus must absorb a neutron before it will split
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What happens when the atom splits?
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Forms 2 new lighter elements that are roughly the same size 2 or 3 neutrons are also released
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What is a chain reaction?
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If the released neutrons move slow enough for another nucleus to absorb them causing more fission
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Where does the energy go when fission occurs?
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Carried away by gamma rays
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How do nuclear weapons work?
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When chain reactions are not controlled using control rods to slow down the reaction and control the amount of energy released — leads to lots of energy being released as an explosion
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What is nuclear fusion?
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Joining small nuclei- opposite of fission 2 light nuclei collide at high speed and fuse to create a larger heavier nucleus E.g. Hydrogen nuclei fuse to make helium nucleus
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What does fusion release lots of?
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Energy

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