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The Discovery of Radioactivity
Wilhelm Roentgen accidentallydiscovered xrays in 1895.
The invisible rays caused photographic plates to darken.
* 1901 Nobel Prize* Element 111 = Roentgenium* a Roentgen = unit of radiation
exposure
Henri Bequerel was studying phosphorescence.* phosphorescent pitchblende darken photographic
plates* noticed that it didn't have to be exposed to light to
do this
Marie and Pierre Curie* took Bequerel's pitchblende sample and isolated what was emitting the invisible rays
* discovered uranium atoms were doing this
Radioactivity: process of emitting rays and particles (radiation)
* later won Nobel prizes * discovered Polonium and Radium
* radioactivity is the spontaneous decay of an unstable iosotope to a more stable one
XAZmass number = p+ and no
atomic number = p+ = IDENTITY!!!
REPULSIVEELECTROSTATIC FORCE
balance between the tworelated to nuclear stability
BAND OF STABILITY* the more protons added, the more neutrons needed to increase the strong nuclear force
Natural Radioactive Decay Modes1. alpha emission2. beta emission3. gamma emission4. positron emission5. electron capture
Many times, one element will change into a brand new element. This is called TRANSMUTATION.
Alpha decay
Try these:
*This is the most common way for a heavy, unstable nucleus to become more stable.
234 92 U
Polonium218
222 86 Rn
Am247
95
Beta decay
With *normal* beta decay:a neutron in the nucleus gets converted into a proton and then
kicks out a beta particle
6328 Ni
14 6 C
* nuclei rich in neutrons do this
Silicon32
Gamma emission = high energy photons* energy comes from the excited nucleus* often accompanies beta decay* several photons with different energy can be emitted* does NOT change A or Z!
Radiation Penetrating Power:
Positron emissionPositron: an electron that has a positive charge
Protons turn into neutron and a positron
K40
electron capture * electron closest to nucleus gets caught* 1s vacancy is filled by higher electrons* Cascade effect* energy released = xray
Po204
Nuclear Stability
* band of stability
* all isotopes containing 84 or more protons are unstable and will decay.
Z n/p20 180 1.5
Isotopes that are neutronrich lieabove the band of stability.Beta emission is common.
Nuclear decay calculations* t1/2 = halflife
* after 10 halflives, the amt of radioactivity drops below detectable levels ("safe")
* Po 213 = 0.0000042 seconds
* U 238 = 4,500,000,000 years
* not a linear process
I 131 is used to treat thyroid cancer.t1/2 = 8 daysHow long would it take to decay to 25% of the original amt?
Remaining amount of Radioactive Element:
N = No (1/2)n
remaining amount initial
amount
# t1/2
can be replaced with t/T where t = elapsed timeT = t1/2
Krypton85 is used in indicator lights of appliances. The halflife of krypton85 is 11 y. How much of a 2.000 mg sample remains after 33 y?
Initial amount =Elapsed time = t1/2 =
N = No (1/2)n
Artificial transmutations:* bombardment of stable nuclei with charged and uncharged particles.* particles are accelerated in a magnetic or electric field
Fermi Lab in Illinois
4 mile circumference
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