Download Nuclear Fission and Fusion Notes

Nuclear Fission and Fusion
Notes
November 3, 2008
What part of the atom is affected by
Nuclear Chemistry?
Do you think the benefits of nuclear
energy outweigh the dangers? Why or
why not?
Goals
To explain the difference between fusion
and fission
To describe half life and solve problems
using half life
An element can have both stable and
unstable isotopes
Ex:
– C-12 is stable so there is no nuclear decay
– C-14 is unstable and radioactive
 A special force called the strong nuclear
force holds the p and the n in the nucleus
together. This force occurs over a very
short distance.
Because of the strong nuclear force, the n
and p attract other n and p.
*n have no charge and they do not repel
each other or the protons.
*p repel each other with the electric force
and attract each other with the strong
nuclear force.
In stable nuclei, the attractive forces
(strong nuclear) are stronger than the
repulsive forces (electric).
Nuclear Fission
Fission- The process by which a nucleus
splits into two or more smaller fragments,
releasing neutrons and energy.
(The production of lighter nuclei from
heavy nuclei)
When U-235 is bombarded by neutrons,
the products of the reaction include 2
lighter nuclei that include Ba and Kr
together with neutrons and energy
Each dividing nucleus releases about 3.2 x
10^-11 joules of energy.
Some of the mass of the particles
before the fission reaction is turned
into energy after the reaction.
matter  energy
mass-energy equation
– E=mc2 Einstein
A small loss of mass results in a great
amount of energy that is given off.
Nuclear Fusion
Fusion – The process in which light nuclei
combine at extremely high temperatures,
forming heavier nuclei and releasing
energy.
Energy is also obtained during the fusion
process
*Stars and Sun: Energy is produced when
H nuclei combine
*We need a large amount of energy to
start a fusion reaction so that the H nuclei
can be brought together and overcome the
electric forces
This energy is provided by the high
temperature
Radioactive Decay Rates
Half life- The time required for half of the
sample of radioactive nuclei to decay.
1st half life  ½ sample remains
unchanged
2nd half life-> ¼ sample remains
unchanged
3rd half life -> 1/8 sample remains
4th half life - > 1/16 sample remains
1. Eventually the entire sample will decay
2. With each successive half life, half of the
remaining sample decays to form
another element.
– Different radioactive isotopes have different
half lives.
– Half lives can last from nanoseconds
billions of years
3. If we know how much of the isotope is
present at the beginning, we can predict
how old the object is.
a. Rocks 40K decays to 40Ar.
b. The ratio of 40K to 40Ar is smaller in older
rocks
( We have more 40Ar than 40K)
c. We can look at the ratios between 14C
and 12C to determine the age of animal
remains or plant remains.
Agenda Items
Chernobyl Article w/ Questions
Half Life problems
– BOTH DUE TOMORROW
 Review for BIG TEST (Due Friday)
4. Calculations with half life
a. 226Ra has a half life of 1599yrs. How long
would it take 7/8 of 226Ra to decay?
b. 131I half life= 8.1days. How long will it take
for ¾ of the sample to decay?
c. Ra 222 has a half life of 3.82days. How
long does it take for 15/16 of the sample to
decay?