1. UNIT 9: Nuclear Chemistry

2. Nuclear Chemistry Natural Radioactivity
-- the changing, or decay, of an atom’s nucleus
-- accompanied by the release of particles and energy
-- changes one element into another (transmutation)
-- occurs in some isotope of every element
-- occurs in all atoms of elements number 83 and above

3. Types of Emissions (Table O)
1. Alpha particle: ( α )
mass = 4 u charge = +2
(2 neutrons, 2 protons
& no electrons)
4He nucleus 2
Example: 238U 
4He +
234Th 92 2 90
-- alpha radiation can be stopped by a few sheets of paper
2. Beta particle: ( β) e
mass = near 0 charge = -1 -1
-- A high speed electron
Notice top numbers and bottom numbers balance!
Example: 14C 
0e +
14N 6 -1 7
-- beta radiation results from a neutron disintegration
1n  1p e 1 -1
-- beta radiation can be stopped by wood or metal

4. See Table N 0e 11B 3. Gamma ray: ( γ ) mass = 0 charge = 0
-- high energy electromagnetic radiation
-- no change to nucleus
-- stopped by lead, thick steel, or high-density concrete
4. Positron: ( β + ) 0e
mass = near 0 charge = +1 +1
Example: 11C  0e +
11B 6 +1 5
-- a positron is the antimatter particle to an electron
The proton : neutron ratio in the nucleus determines the decay mode
too many neutrons
beta
too many protons positron
very large nuclei alpha
See Table N

5. Emissions can be separated by:
1. Testing their penetrating power
2. Seeing how they respond to an electric field

6. Every radioisotope follows a decay series, until it becomes
a stable (non-radioactive) atom
Example: U-238 to Pb-206

7. Half - Life
the amount of time it takes for half the atoms of a radioisotope
to decay
U-238  4.5 billion years Po-214  seconds
See Reference Table N
Example: if you have 100 grams of Cs-137:
after 30.2 yr, 50 grams remain unchanged
after 60.4 yr, 25 grams remain
after 90.6 yr, 12.5 grams remain

8. Example: A company has 96 grams of Sr-90, and wants to dump it
into the river. Legally they can only dump 3 grams. How
long must they wait until only 3 grams are left?
Solution: Half-life Sr-90 = 29.1 yrs
Time Amount Left g
29.1 yr g
58.2 yr g
87.3 yr g
116.4 yr g
145.5 yr g

9. The amount of C-14 is constant (formation = decay)
The C-14 enters living things
When an organism dies, it no
longer takes in C-14, and the
existing C-14 decays ...
half-life = 5715 years
So, after 5715 years, bones have half as much C-14 as living things
After about 10 half-lives, too
little C-14 is left to measure
The changing of U-238 to Pb-206
(half-life 4.5 billion years) can be
used to find the age of rocks
5715
22,860

10. Biological Effects of Radiation
In low doses, radiation primarily affects the DNA of living cells
-- it can change them into cancer cells
The effect is greatest on dividing cells:
1. Blood cells leukemia; anemia
2. Skin cells -- skin cancer
3. Sex cells -- }
birth defects
4. Unborn babies --
5. Cancer cells -- radiation therapy attempts to destroy cancer
cells without harming healthy cells
Co-60 is used in cancer treatment
I-131 is used to treat thyroid disorders

11. Radioisotopes can also be used:
-- as tracers to study chemical or biological reactions
-- to kill bacteria and sterilize food and other items
Look for this symbol:
-- to measure the thickness of various materials

12. Radioisotopes can be made artificially in an accelerator
Accelerators use magnetic or electric fields to speed up a charged
particle, and steer it into a collision with another atom --
4He Al  30P n
alpha ordinary radioisotope neutron
particle aluminum of phosphorus
atom
-- causing an artificial transmutation (artificial means people did it)
Example: A scientist uses a particle accelerator to collide an
alpha particle with a nitrogen-14 atom. A proton is produced, along
with one other atom. What’s the other atom?

13. Nuclear Fission -- is the breaking of a large nucleus into pieces
-- accompanied by the release of lots of energy
-- occurs when a fissionable nucleus, usually U-235 or Pu-239
is hit by a particle such as a neutron

14. Top numbers and bottom numbers always balance:
Protons:
92 =
Mass: =

15. The neutrons released may be absorbed by another material,
or they may strike another fissionable nucleus
If enough other fissionable nuclei are present (a critical mass), the process continues -- a chain reaction
If the mass of fissionable nuclei is less (subcritical),
the reaction stops
If the mass is very large (supercritical), a nuclear explosion results

16. Natural uranium is 0.7% U-235; not enough for a chain reaction
It must be enriched to reach a critical mass
mass of fuel is supercritical
95% U-235
mass of fuel is critical
3% U-235

17. Electricity from Nuclear Fission -- Nuclear Power
A fission reaction generates heat,
--which turns water to steam
--which generates electricity as it spins a turbine

18. Advantages Disadvantages The Reactor Core consists of:
--Fuel rods (Uranium, enriched for U-235; or Plutonium)
--Control rods (Boron or Cadmium) absorb neutrons to stop
or slow the chain reaction
Advantages
Disadvantages
No air pollution problems
(acid rain, global warming, etc)
Chance of radiation release
accident
Does not use up fossil fuels—
less dependence on foreign
oil sources
Used fuel (nuclear waste)
remains dangerous for years
May allow spread of nuclear
weapons
3. Safer (?)
4. Cost (?)
4. Cost (?)

19. Nuclear Fusion 2H + 3H  4He + 1n
--the combination of small nuclei to form larger atoms
Example:
2H H  4He + 1n
Deuterium + Tritium
Two isotopes of Hydrogen
-- releases even more energy than fission ( E = mc2 )
-- provides the energy for the sun and other stars
Fusion could potentially serve as a power source:
-- uses hydrogen for fuel ( from water)
-- produces almost no radioactive wastes

20. BUT --
Fusion requires very high temperatures and pressures to
overcome the repulsion between nuclei ( + and + )
-- about 10,000,000 ºC
Producing these temperatures and containing the hot gases
are problems yet to be solved