Atoms

Atoms

1. Life Cycle of Stars

   • clouds of DUST & GAS - gravity makes gas/dust spiral together = protostar
     • gravitational energy converted to heat energy = temp. rises - when temp.'s high enough, hydrogen nuclei - nuclear fusion = helium nuclei = heat & light
       • long stable period - heat created = outward pressure to balance gravity pulling inwards - millions of years - massive amounts of hydrogen = MAIN SEQUENCE STARS
         • eventually hydrogen = runs out - stars swells = RED GIANT (small star) - RED SUPER GIANT (big star)
           • small-medium star (like Sun) - becomes unstable & ejects outer layer of dust & gas as PLANETARY NEBULA
             • hot dense solid core = WHITE DWARF - cools down = BLACK DWARF - disappears
           • big stars - glows brightly again - fusion & expand/contract several times = heavy elements like iron in nuclear reactions - explode in SUPERNOVA (=elements heavier than iron) - ejects to form new planets & stars
             • exploding supernova - throws outer layer of dust & gas - very dense core - NEUTRON STAR - star = big enough = BLACK HOLE

2. HALF-LIFE = AVERAGE TIME it takes for the NUMBER OF NUCLEI in RADIOACTIVE ISOTOPE SAMPLE to HALVE

   • time it takes for count rate to halve

3. Nuclear Fission & Fusion

   • Nuclear Fission - Splitting Up - big nuclei
     • nuclear power station - nuclear reactors - uranium-235/plutonium-239
       • chain reaction = nuclei split up & release heat energy - heats water - steam turbine - generator
   • Chain Reactions
     • slow moving neutron - absorbed by nucleus - unstable nucleus = split
       • uranium/plutonium splits up - two/tree neutrons released = hit another nucleus = split = chain reaction
         • two new smaller nuclei = radioactive - "wrong" no. of neutrons
           • splitting (fission) = lots of energy
   • Nuclear Fusion - Joining - small nuclei
     • light nuclei (e.g. hydrogen) - join
       • releases lots of energy (- more than fission)
     • energy from stars - from fusion
     • less radioactive waste - plenty of hydrogen as fuel
     • big problem = high temp. - 10 million°C
       • need strong magnetic field - to hold hydrogen at high temps/pressure

4. Ionising Radiation

   • radioactive isotopes - decay into other elements & give out radiation
   • Radioactive = Random
     • radioactive substances - give out radiation from nuclei - no matter what's done to them
     • can't say when any one of them will decay & can't make decay happen
       • unaffected by physical conditions - temp. & chemical bonding
     • spit out - alpha/beta/gamma
   • Background Radiation - present always
     • 1. unstable isotopes - all around us - air, food, building materials & rocks
     • 2. space - cosmic rays - the Sun
     • 3. man-made sources - nuclear accidents / dumped nuclear waste
   • Alpha Particle = helium nucleus - big/heavy/slow moving
     • don't penetrate far - stopped quickly - even in air
     • strongly ionising - bash into & knock electrons off
   • Beta Particle = Electrons - quite fast
     • penetrate moderately - long range in air - moderately ionising
     • for each b-particle - neutron → proton
   • Gamma Rays = Short EM Waves
     • penetrate far into materials - straight through air
     • weakly ionising - only pass through (don't bash)
   • Radiation Damage Depends on Radiation Dose = higher risk of cancer
     • underground rocks - can release radioactive radon gas - gets trapped in houses
     • nuclear industry & uranium miners - 10 times - protective clothing/face masks - no touch/inhale - monitor with regular check-ups
     • radiographers - lead aprons - stand behind lead screens - no prolonged exposure
     • high altitudes (- jet planes) - cosmic rays - pilots risk
   • Alpha&Beta - Deflected by Electric & Magnetic Fields
     • alpha = larger charge & feel greater force - deflected less - greater mass

5. Atomic Structure

   • J J Thompson - "plum pudding" - atoms = spheres of positive charge with electrons stuck in them
     • Rutherford & Marsden - fired alpha particles at thin gold foil - positively charged alpha would slightly deflect
     • most went straight through but few bounced back = shocking
       • most atom mass = centre in nucleus most atom = empty space
   • Nuclear Model
     • radius of nucleus = 10 000 times smaller than atom radius

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