Atomic Structure-P4

Atomic Structure-P4

1. The model of the atom

   • 1804=Dalton agreed with Democritus that matter was 'tiny spheres'
     • 100 years later=Thomson discovered electrons,suggested that atoms were spheres of positive charge(Plum pudding model)
       • 1909-Rutherford fired alpha particles at thin gold foil(alpha scattering experiment)/Almost all particles went through the sheet but some were deflected
         • Because they were deflected back,people realised that most mass of the atom was concentrated at the centre(Nucleus/positive charge)
   • Bohr said that electrons oribiting the nucleus go at certain distances(energy levels)
     • Evidence from experiments changed the model to have a nucleus made up of particles(protons) which had positive charges that added up to the overall charge
       • 1932-Chadwick proved existence of neutrons
         • Current model=Protons=+1 charge/neutrons=0 charge/Electron=-1/Rest of atom is empty space/1 X 10^-10m
           • Number of protons=Number of electrons
             • Electrons can move/if they gain energy they move to a higher energy level
               • 1+ electrons leave atom=Atom becomes positively charged

2. Isotopes

   • Mass number=Protons+Neutrons
   • Isotopes=Atoms with same number of protons but a different number of neutrons
   • Some isotopes are unstable-they decay into other elements and give of radiation to become stable again(Radioactive decay)
     • Radioactive substances spit out ionising radiation from nucleus
       • Ionising radiation=Knocks electrons of atoms(creates + ions
         • Alpha/Beta/Gamma

3. Nuclear radiation

   • Gamma-Waves of electromagnetic radiation released by the nucleus
     • Penetrate into materials/travel a long distance
       • Weakly ionising=tend to pass through rather than collide with atoms
         • Absorbed by thick sheets of lead or concrete
   • Beta-Fast moving electrons released by nucleus/no mass/Charge of -1
     • Moderately ionising/Absorbed by aluminium
       • For every beta particle=Neutron in nucleus has turned into a proton
   • Alpha=Particles are helium nuclei
     • two neutrons and two protons(like helium)
       • Don't penetrate far/absorbed by sheet of paper
         • Strongly ionising

4. Nuclear equations

   • Mass and Atomic number have to balance
     • Atom before decay=atom after decay+radiation emmited
   • Alpha decay decreases the charge and mass of the nucleus
   • Beta decay increases the charge of the nucleus
     • Neutron turns into a proton and releases an electron
   • Gamma rays don't change the charge or mass of the nucleus

5. Half-Life

   • Measure half-life using a graph(activity against time)
   • Half life=Time taken for the number of radioactive nuclei in an isotope to half
     • Time taken for activity(count-rate) to halve
       • Short half life=activity falls quickly/dangerous because of the high amount of radiation they emit but then become safe
         • Long half life=Activity falls slowly because most of nuclei doesn't decay for a long time/releases small amounts of radiation for a long time(dangerous as places are exposed for years)
       • Activity is measured in Becquerels,Bq(1Bq=1 decay per second)
   • Use a Geiger-Muller tube and counter to measure count-rate of radiation per second

6. Background radiation and contamination

   • Background radiation comes from many sources:Radiation from space(cosmic rays)/Radiation from human activity=nuclear waste=tiny proportion
     • Exposure to radiation=Irradiation
       • Irradiating something doesn't make it radioactive
         • Keeping things in lead-lined boxes,stand behind barriers and standing far away reduces the effects of irradiation
   • Contamination-Radioactive particles getting onto objects
     • Dangerous when particles get inside your body
       • Gloves and tong should be used when handling this/Or protective suits to stop breathing it in
         • Outside the body-beta and gamma are most dangerous=penetrate body and get to organs
           • Inside body-Alpha is dangerous because damage in done in localised areas
             • Gamma is least dangerous inside as it passes straight out

7. Uses and risks

   • Radiation=Enter living cells and damage tissue/low doses tend to cause minor damage without killing cells=mutant cells can divide and cause cancer/higher doses=kill cells completely=radiation sickness
     • Gamma sources are used in medical tracers because radiation passes out of the body without causing ionisation
       • Have a short half-life to the radioactivity quickly dissapears
         • Radiotherapy=Gmma rays are directed carefully to kill cancer cells however it can do damage to normal cells
           • Prolonged exposure causes risks and side effects but it can get rid of cancer all together so it is worth it
             • Perceived risks=How risky a person thinks something is

8. Fission and fusion

   • Nuclear fission=Splitting a large,unstable nucleus
     • Uranium or plutonium
       • Rarely happens=Nucleus has to absorb a neutron before it splits
         • Atom splits=Forms two lighter elements(same size)
           • 2/3 neutrons are released
             • If any neutrons are moving slow enough to be absorbed then there is more fission(chain reaction)
               • Uncontrollled chain reactions leads to lots of energy being released as an explosion(nuclear weapons)
   • Nuclear fusion=Two light nuclei collide at high speeds and join to create a large/heavy nucleus(hydrogen fuse to create helium nucleus
     • Heavier nucleus=not as much mass as the 2 light nuclei did/lighter nuclei=mass converted to energy=energy is released as radiation