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Chapter 7:
Radiation
Remember from Chemistry:
• Mass number: the number of protons +
number of neutrons
• Atomic number: the number of protons
in an atom of an element.
• Isotopes: atoms of the same elements
that have a different number of neutrons
and therefore a different mass number
There are many isotopes of carbon.
• For example: carbon-12, carbon-13 and
carbon-14
• How many protons in each of these atoms of
carbon?
– 6
• How many neutrons in each of these atoms of
carbon?
•
Carbon-12 = ____ neutrons
•
Carbon-13 = ____ neutrons
•
Carbon-14 = ____ neutrons
•
Carbon-12 = 12 C
6
•
Carbon-13 = _____
•
Carbon-14 = _____
Mass number
Atomic number
Isotopes of Hydrogen
• Hydrogen: 1 proton and 0 neutrons
• Deuterium: 1 proton and 1 neutron
• Tritium: 1 proton and 2 neutrons
Radioactive Decay:
• The nuclei of some isotopes are stable,
whereas, others are not.
• In unstable nuclei, the nuclear force is not
enough to overcome the repulsive
electrical forces with in the nucleus.
• As a result the nuclei are radioactive.
• When an unstable nucleus emits radiation,
it undergoes a radioactive decay
Types of radioactive decay
• Alpha Decay:
• Particle emitted: Helium nucleus
• Result: Atom with two fewer protons and
neutrons
• U → Th + He
• U → Th +
Types of radioactive decay
• Beta
• Particle emitted: Electron
• Result: # of neutrons decrease by one and
the # of protons increase by one = new
element
• C → N + e
• C →
N + β
Types of radioactive decay
• Gamma
• Particle emitted: No particle, but gives off
excess energy
• Result: No change to type of nucleus, it just has
less energy.
• Ba → Ba + Gamma radiation
• Ba → Ba +
Assignment:
• Fill in the 7.1 worksheets p125 (all) and
126 (odd numbers)
Half Life:
• Half Life: The time required for half the nuclei of
a sample of a radioactive isotope to decay.
• The half life is the same regardless of the
starting amount of the isotope.
• Because the amount of all radioactive isotopes
decrease by half in each half life, graphs that
show the amounts of these isotopes at various
times look similar.
• The only difference is the time scale for
decrease.
Examples of half life:
120
100
80
60
40
20
0
carbon
0
Uranium
20000 40000 60000 80000
Time (Years)
sample of uranium
amount of sample
left
carbon
600
500
400
300
200
100
0
Uranium
0
50000
100000
Time (years)
150000
Half Life Table for
Solving Word Problems
Time
# of Half lives
Amount of
parent isotope
** optional **
Amount of
daughter
isotope
0
0
Original Amount
0
1
2
3
4
Practice Problems
Homework:
• Fill in the 7.2 worksheets p134 and 135