Transcript 11. Radioactive Decay

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What are atoms made of?
Atoms are made of three basic building blocks called
protons, neutrons and electrons.
There are two properties of protons, neutrons and electrons
that are especially important: mass and charge.
Particle
Mass
Charge
proton
1
+1
neutron
1
0
electron
almost 0
-1
In any atom, the number of electrons is equal to the number
of protons and so the overall charge of an atom is zero.
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Particles in the modern model
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What makes a carbon atom carbon?
The atoms of any particular element always contains the
same number of protons.
Carbon atoms always have six protons. Atoms with different
numbers of protons must be other elements. For example:
 all atoms with 1 proton are hydrogen atoms;
 all atoms with 17 protons are chlorine atoms.
In the periodic table, there are two numbers found with each
element. What do these numbers represent?
Mass number is the number of
protons + the number of neutrons.
Atomic number (or proton
number) is the number of protons
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What are isotopes?
All carbon atoms have the same number of protons, but
not all carbon atoms are identical.
Although atoms of the same element always have the same
number of protons, they can have different numbers of
neutrons. Atoms that differ in this way are called isotopes.
For example, carbon exists as three different isotopes:
carbon-12, carbon-13 and carbon-14:
mass number
is different
atomic number
is the same
Potassium is another element that exists as three different
isotopes: potassium-39, potassium-40 and potassium-41.
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Atomic structure – key words
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Types of radioactive decay
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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.
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What happens during beta decay?
An 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.
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What happens during gamma decay?
Gamma radiation is a form of electromagnetic
radiation, not a type of particle.
When an atom’s nucleus decays and emits gamma radiation,
it releases energy in the form of electromagnetic radiation.
There is no change to the make up of the nucleus and
so a new element is not formed.
Gamma rays are usually emitted with alpha or beta particles.
For example, cobalt-60 decays releasing a beta particle. The
nickel formed is still not stable and so emits gamma radiation.
60
27Co
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
60
28Ni*
+
β

60
28Ni
+

The nickel does
not change into
a new element.
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Radioactive decay – true or false?
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How can radioactivity be measured?
Radioactivity cannot be seen, it has no smell and does not
make any sound so how can it be detected?
Radioactivity can be
detected with a Geiger
counter, which is a
Geiger-Müller (GM) tube
connected to a ratemeter.
It can also be used to
measure the amount of
radiation.
GM tube
ratemeter
The ratemeter gives a reading in ‘counts per second’ and
a loudspeaker ‘clicks’ for each particle, or burst of radiation,
detected by the GM tube.
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What happens to radioactivity?
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What is half-life?
Radioactive decay is a spontaneous process that cannot be
controlled and is not affected by temperature.
However, each radioactive element has its own particular
decay rate, which is called the half-life.
The half-life of a radioactive element is the time
that it takes half the atoms in a sample to decay.
For example, the half-life of the isotope iodine-131 is 8 days.
This means that after 8 days half the atoms in a sample of
iodine-131 have decayed. 8 days later half the remaining
atoms have decayed and so on.
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How is half-life calculated?
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How long are half-lives?
Half-lives range from
millionths of a second
to millions of years.
Some types of nuclei
are more unstable than
others and decay at a
faster rate.
Radioisotope
Half-life
boron-12
0.02 seconds
radium-226
uranium-235
1602 years
710 million years
Xenon-133 is a radioactive isotope used for studying lung
function. Why does its half-life of 5.2 days make it suitable
for this use?
Uranium-235, which is used in nuclear reactors and nuclear
weapons, has a half-life of 710 million years. Why is the use
of uranium-235 considered controversial?
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What is the half-life of carbon-14?
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How does carbon dating work?
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What are the problems of using carbon dating?
What are some of the problems with using carbon dating
to predict the age of a sample?
 The half-life of carbon-14 is 5,700 years. If the sample is
older than 60,000 years, the amount of carbon-14 is too
small to measure accurately. Instead, radioactive isotopes
with longer half-lives, such as uranium-235 with a half-life
of 710 million years, can be used to date older samples.
 Samples can become contaminated with materials of a
different age which may confuse the readings of carbon-14.
 Carbon dating anything that died after the 1940s, when
nuclear bombs, nuclear reactors and open-air nuclear tests
began, is harder to date precisely due to contamination
from this increased background radioactivity.
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Using half-life to date a sample
Half-life can be used to do many useful calculations.
For example, the half-life of carbon-14 is
5,700 years. If a fossil bone has a count of
25, and a piece of bone from a living body
has a count of 200, how old is the fossil?
 After one half-life, the count will decrease by half to 100.
 After the second half-life, the count decreases by half
again to 50.
 After the third half-life, the count decreases to 25.
 Three half-lives of carbon-14 have passed,
so 3 x 5,700 years makes the fossil 17,100 years old.
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Using half-life in calculations
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Glossary
 atomic number – The number of protons in the nucleus
of an atom, which is the same for all isotopes of an element.
 half-life – The time taken for the number of radioactive
atoms in a sample, or the count rate, to decrease by half.
 isotopes – Different forms of the same element, with the
same number of protons but different numbers of neutrons.
 mass number – The total number of protons and
neutrons in the nucleus of an atom, which differs for each
isotope of an element.
 radioactive decay – The breakdown of unstable
radioactive nuclei by releasing radiation.
 radioisotope – A radioactive isotope of an element,
which may be naturally occurring or artificially created.
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Anagrams
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Multiple-choice quiz
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