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Nuclear Chemistry. Electromagnetic Radiation. All objects give off radiation Radioactive unstable nuclei (more neutrons than protons) decay to different element with different atomic number Most atoms remain unchanged Common properties of EMR form of energy and has no mass

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Electromagnetic radiation l.jpg
Electromagnetic Radiation

All objects give off radiation

Radioactive

unstable nuclei (more neutrons than protons)

decay to different element with different atomic number

Most atoms remain unchanged

Common properties of EMR

  • form of energy and has no mass

  • travels at the speed of light (3 X 108 m/s)

  • can travel through a vacuum; does not need a medium like H2O

  • emitted by atoms as they decay or after they are energized

  • moves through space as packets of energy called photons

  • energy of photons is related to the frequency


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Two types of radiation

Ionizing radiation

x-rays, gamma rays

greatest energy

high frequency; short wavelength

cause particles like electrons to be ejected

exposure can cause great damage to tissues

Non-ionizing radiation

radio waves, microwaves, infrared, visible light, ultraviolet

lower energy

low frequency; long wavelength

exposure does not cause great damage to tissues


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Atoms

Particle location charge mass

proton p+ nucleus +1 1 g/mol

neutron no nucleus 0 1 g/mol

electron e- outside nucleus -1 0

in energy levels


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Atomic number

= number of protons

= number of electrons if atom is neutral

identifies the element

Mass number

= number of protons + neutrons


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Isotopes

Atoms of the same element having different number of neutrons

Radioisotope

Radioactive (ionizing)

Decays spontaneously

Symbol mass # 90

Sr

atomic # 38

Name

Add mass number to the name of the element or symbol

Example: strontium-90 or Sr-90


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Stability of isotopes

stable isotopes do not decay spontaneously

1500 known isotopes and only 264 are stable

85% of all isotopes are unstable

can be predicted using proton to neutron ratio

nuclei > atomic number 84-alpha


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Radioactive decayemission of radiation is one way that an unstable nucleus is transformed into a more stable one with less energy

Alpha emission 

alpha particle is composed of 2 protons and 2 neutrons

nucleus of helium-4 atom

more massive than beta particle

poor penetrating power

slow speed

potential to cause great damage to tissue

produces new particle with lower atomic # and mass #

symbols of alpha particle

42He     and    

example of alpha decay

22688Ra   42He   +    22286Rn


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Beta emission

beta particle is an electron

beta emission is equivalent to the conversion of a neutron to a proton

smaller than an alpha particle

moves faster and penetrates better than alpha

produces new particle with higher atomic # and same mass #

symbols of beta decay

0-1e      and      0-1      and       

example of beta decay

146C   0-1e    +    147N


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Gamma

gamma is a form of electromagnetic radiation

high energy photons

represents energy lost when the remaining nucleons reorganize into

more stable arrangements

moves at speed of light

not a particle

penetrates best of all types of radiation

no change in particle that undergoes gamma decay

symbols

00     and       


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Positron emission 01e

positron is a positive electron

produces a new particle with lower atomic # and same mass #

positrons have a very short life because it is annihilated when it collides with an electron, producing gamma rays

01e   +    0-1e       2 00

example of positron  emission

3819K   3818Ar   +    0+1e


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Electron capture + 0-1e

only type of radioactive decay in which the particle is on the reactant

side of the equation (electron is consumed rather than formed)

electron

converts a proton to a neutron

11p   +   0-1e   10n

example of electron capture

10647Ag   +   0-1e   10646Pd


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Half-life

Rate of decay of radioisotopes

Time required for half the atoms of a radioactive nuclide to decay

rate of decay = # of atoms that disintegrate per time

A = kN

where A  =  activity  =  # disintegrations /time

k  =  decay constant   (specific to isotope)

N  =  number of atoms

Determined by

ln    Nt   =   -kt              ln = natural logarithm

No

When t = half-life the t = t1/2     and   t1/2  =  0.693

k    

Nt  =   # radioactive atoms after time t

N0  =  # radioactive atoms at time 0


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Artificial Radioactivity (bombardment reactions)

make a ‘new’ element by bombarding an element with a particle

Four particles involved

  • target nucleus is the stable isotope that is bombarded

  • projectile (bullet)  is the particle fired at the target nucleus

  • product is the heavy nucleus produced in the reaction

  • ejected particle is the light nucleus or particle emitted from the reaction

    example:

    target nucleus   +  projectile     product nucleus + ejected particle

    2713Al             +        42He       3015P             +        10n


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Nuclear Energy

In order to produce energy, an atom must lose mass.

Nuclear reaction releases1 million times more energy than chemical reaction

Atoms before iron will undergo fusion to produce energy

Atoms after iron will undergo fission to produce energy

Iron is the “nuclear sink”  It will not undergo fusion or fission.


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Fission

advantage- can do it now

disadvantage-fuel hard to get and produces waste products that are radioactive

chain reaction-reaction continues because an ejected particle form the original reaction can split more nuclei

critical mass-minimum volume of fissional material necessary to keep a chain reaction going

Nuclear power plants

produce electricity  (nearly 20% of U.S. needs; 110 plants)

produce heat to boil water to make steam to turn turbines

Parts of power plant

  • fuel rods-pellets of uranium dioxide

  • control rods-absorb neutrons to control rate of reaction

  • moderator-slows down high-speed neutrons

  • generator-produces electricity

  • cooling system-cools steam


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Fusion

involves formation of a new, more massive atom by forcing two less-massive nuclei to combine

powers the sun and stars

advantage-produces little waste and fuel is readily available

disadvantage- can’t do it on large scale yet

energy released is enormous and can be calculated using Einstein’s theory of relativity   E  =   mc2


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Risks and Benefits of nuclear chemistry

Benefits

  • Energy source

  • Tracer studies-isotopes used to trace systems

    • medicine-  find and treat diseases

    • petroleum pipelines

    • agriculture

  • Irradiation –sterilization

    • medicine

    • prevent spoilage


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Risks

  • 4 possible biological effects

    • radiation can pass through with no damage to cells

    • radiation can pass through with damage which the cells repair

    • radiation can pass through with damage that the cells cannot repair

    • radiation can kill the cells

  • Exposure

    • fallout from nuclear weapons testing

    • increased exposure to cosmic radiation during air travel

    • radioisotopes released into the environment from nuclear power and

    • and other nuclear technologies

  • Nuclear wastes


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