Transcript Radioactivity, Fission and Fusion

Radioactivity, Fission and Fusion
Revise the following:
Sources of background radiation
Basic atomic structure
Isotopes
History of the atom  Modern atom
Effect of alpha and beta radiation on
nuclei
• Nuclear Fission
• Nuclear Fusion
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Sources of background radiation
A reminder about atoms - complete
• Atoms may lose or gain ________to
form charged particles called ions.
• All atoms of a particular element have
the _____ number of protons.
• The total number of protons in an atom
is called its _______number.
• The total number of protons and
neutrons in an atom is called its
_____number.
Early ideas about the atom:
The plum pudding model
• A scientist called JJ Thomson suggested that an atom
is a positively-charged sphere with negative electrons
distributed throughout it
• Thomson’s model became known as the plum pudding
model, because the electrons in the atom were thought
to be like raisins in a plum pudding.
Rutherford, Geiger and Marsden:
The nuclear model of the atom - 1
Rutherford, Geiger and Marsden:
The nuclear model of the atom - 1
Rutherford, Geiger and Marsden:
The nuclear model of the atom - 1
Rutherford, Geiger and Marsden:
The nuclear model of the atom - 1
The modern atom: The nuclear model
The nucleus is where
most of the mass of
the atom is found.
It contains protons and
neutrons.
The electrons orbit
the nucleus in layers
called shells.
• In an atom the number of electrons is equal to the
number of protons in the nucleus.
• The atom has no net electrical charge
Plum pudding Vs nuclear model
Plum pudding
Nuclear model
Atom is solid sphere
Atom is mostly empty space
Negative electrons
distributed throughout the
atom
Positive charge throughout
the atom
Electrons orbit the nucleus
No nucleus
Contains a nucleus
Positive charge
concentrated in the centre
Isotopes
Alpha radiation
What happens during alpha decay?
An alpha particle consists of two protons and
two neutrons. It is the same as a helium nucleus.
When an atom’s nucleus decays and releases an alpha
particle, it loses two protons and two neutrons.
mass number
decreases by 4
238
U
92
234
Th
90
+
4
α
2
atomic number
decreases by 2
The number of protons has changed, so the decayed atom
has changed into a new element.
Beta radiation
What happens during beta decay?
A beta particle consists of a high energy electron, which
is emitted by the nucleus of the decaying atom.
When an atom’s nucleus decays and releases a beta particle, a
neutron turns into a proton, which stays in the nucleus, and a
high energy electron, which is emitted.
mass number
remains the same
14
C
6
14
N
7
+
β
atomic number
increases by 1
The decayed atom has gained a proton and so has changed
into a new element.
Nuclear fission
fission
+
Uranium
235
+
+
neutron
uranium
236
Strontium
90
+
+
Xenon
144
+
neutrons
• Nuclear fission is the splitting of an atomic nucleus
• There are two fissionable substances in common use in
nuclear reactors, uranium 235 and plutonium 239
• For fission to occur the uranium 235 or plutonium 239
nucleus must first absorb a neutron
• The nucleus undergoing fission splits into two smaller
nuclei and 2 or 3 neutrons and energy is released
• The neutrons may go on to start a chain reaction
What is a chain reaction?
Nuclear fission results in a chain
reaction because each time a nucleus
splits it releases more neutrons, which
can go on and cause more fission
reactions to occur...
and so on.
+
This is why a chain reaction
releases a lot of energy so rapidly.
If a chain reaction is uncontrolled, heat builds up
very quickly.
A chain reaction must be controlled to maintain a
steady output of heat.
Nuclear fusion
+
deuterium
+
+
tritium
fusion
helium
+
neutron
• Nuclear fusion is the joining of two atomic
nuclei to form a larger one.
• Nuclear fusion is the process by which
energy is released in stars