Transcript KS4 Half life and model of the atom

Radioactive decay
Learning Objectives
•
Understand what happens to the nuclei of
radioactive atoms during alpha, beta and gamma
decay.
•
Know the definitions of “half-life” and how to
perform simple calculations using it.
•
Understand the basic principles of nuclear power
generation.
What we used to think…
It was believed that atoms were:
1. Spheres of positive charge.
2. With negative charges spread
through it.
This resembled a plum-pudding, so
it was called the ‘Plum –pudding’
model.
This was wrong!
How did we discover current ideas
about the structure of the atom?
Rutherford’s team:
Ernest Rutherford and his team of scientists performed a
famous experiment in Manchester:
They fired some alpha particles at a piece of thin gold foil
(only a few atoms thick):
If the ‘Plum Pudding’
model of the atom was
correct, the alpha
particles should pass
straight through and only
be slightly deflected.
This did not
happen.
What Rutherford’s team observed……..
1. Most of the alpha particles went straight through
the foil.
2. Some alpha particles were deflected through large
angles.
3. A very few alpha particles were reflected straight
back.
Rutherford’s conclusions
Observation
Most alpha particles went
straight through the foil.
A few were deflected
through large angles.
A very few were reflected
straight back.
Conclusion
Atoms are mostly space.
The nucleus is very small
compared to the size of
the atom and it contains
most of the mass and all
the positive charge.
Radioactive Decay
Why is it that there are different types of radiation?
What is going on inside the nucleus?
The three types of decay are………
Alpha
Gamma
Beta
Type of decay:
What is emitted?
Alpha decay
Alpha particle (helium nuclei)
Description of decay:
Example of decay:
2 neutrons and 2 protons are
emitted from the nucleus.
238
92
Effect on A and Z:
U 
234
4
90
2
Th +  + energy
A decreases by 4, Z decreases by 2
(A-4, Z-2)
Beta decay
Type of decay:
What is emitted?
High energy electron
Description of decay:
Example of decay:
A neutron in the nucleus decays
into a proton and a high energy
electron which is emitted.
14
C 
6
Effect on A and Z:
14
0
N +  + energy
7
-1
A stays the same, Z increases by 1
(A=, Z+1)
Type of decay:
What is emitted?
Description of decay:
Effect on A and Z:
Gamma decay
High energy electromagnetic radiation.
Nucleus changes shape into a
more stable shape. Gamma
radiation emitted as a result.
A stays the same, Z stays the same
(A=, Z=)
Half life
There are two definitions of half life:
The time it takes the number of radioactive
nuclei in a sample to decrease by 50%.
The time it takes the count rate from a
radioisotope to decrease by 50%.
You must learn both of these definitions!
Graphical representation of half life
Decay rate
(counts/min)
What is the half life of the radioisotope
represented by the following graph?
80
The time it takes the count rate to
decrease from 80 per min to 40 per min is
what? 2 mins
60
40
Double check, the time it takes
the count rate to decrease from
40 per min to 20 per min is?
20
2 mins
2
4
6
8
Time (min)
The half life of the radioisotope is 2 mins.
Half life questions
1. What are the two definitions of half life?
2. If 1/64th of an original radioisotope is left after 1
hour, what is the half life of the sample?
3. A radioisotope has a half life of 12 minutes. What
fraction of the radioisotope will be left after 2
hours?
4. The background radiation in a laboratory is 13
counts per minute. The count rate from a
radioisotope is measured and it has a reading of
119 counts per minute. If the half life of the
radioisotope is 10 minutes, what will be the reading
20 minutes later?
Carbon Dating
All living things take in a little radioactive carbon-14
in photosynthesis, as well as the normal carbon-12.
When living things die, they stop taking in carbon-14
and so the carbon-14 present at death slowly
decays to carbon-12 (half-life is 5 600 years).
The radioactivity due to the decay of carbon-14 can
be used to date bones, wood, paper and cloth.
Example
A fresh bone gives a radioactive count of 170 counts
per minute. Another ancient bone of the same mass
gives a count rate of 50 counts per minute. The
background count is 10 counts per minute.
How old is the bone?
click
Counts due to bones are 170 - 10 = 160 (fresh) and
50 - 10 =40 (ancient)
The count rate of the carbon-14 has fallen to one quarter of
its original value, i.e. 160/2 = 80, 80/2=40.
This is two half lives,
So the bone is 5600 x 2 =11200 years old.
Nuclear Power
When a nucleus decays it gives out heat energy.
In a nuclear power station, the uranium-235 atoms decay
and give out energy and neutrons.
Each time a uranium atom splits it produces 2 or 3
neutrons (depending on the reaction). These go on to hit
other uranium atoms, which causes them to decay. A
chain reaction is set up where more and more energy is
released. In a nuclear reactor the process is carefully
controlled so that neutrons are absorbed harmlessly and
the energy released is controlled.
In a nuclear bomb the reaction is not controlled, and the
bomb explodes!
Nuclear Power - fission
Fast neutron from previous decay cause the Uranium nucleus to split.
Nuclear Power
Kr
n
n
n
Uranium
In the reaction above a
neutron from a previous
decay can lead to more
and more decays.
This is called a chain reaction.
n
Ba
More
decays
Radioactive waste
Radioactive waste from nuclear power stations is an
environmental concern. The problem is the waste
stays radioactive for thousands of years.
The current solutions are:
1. Store it at the nuclear power station until is filled up.
2. Dump it far out at sea.
3. Store it deep underground in non-permeable rock.
Radiation questions
1. What are the three types of radiation?
Alpha, beta and gamma
2. Which type of radiation is the most penetrating?
Gamma
3. Why is radioactive waste not stored in permeable rock?
It could contaminate water that seeps through the rock.
4. Why should nuclear power stations not be situated in
geologically active regions?
Earthquakes could cause radioactive spills.
During alpha decay, which of the following is true?
A.
Relative atomic mass increases by 2
B.
Relative atomic mass decreases by 2
C.
Relative atomic mass increases by 4
D.
Relative atomic mass decreases by 4 
During beta decay, which of the following is true?
A.
Atomic number increases by 1 
B.
Atomic number decreases by 1
C.
Atomic number increases by 2
D.
Atomic number decreases by 2
What fraction of a radioactive sample is left after
4 half lives?
A.
1/2
B.
1/4
C.
1/8
D.
1/16
