Transcript Radioactive Decay

Radioactive Decay
When elements have unstable nuclei, they
decay, forming more stable nuclei and giving
off energy. In this lesson, you will learn what
changes happen with each kind of decay.
Radioactive Decay
Unstable nuclei decay in
order to become stable.
http://www.hko.gov.hk/education/dbcp/radiation/eng/image/unstable_nuclear.gif
What makes unstable
nuclei unstable?
They are unstable
because the electrostatic
repulsion between the
protons is near to or
greater than the strong
nuclear force holding the
nucleus together.
This happens when there
is an unfavorable balance
of protons and neutrons
OR when the nucleus is
just too large.
Radioactive Decay
The largest stable nucleus is Bismuth-209.
Every atom whose mass is more than 209
a.m.u. is unstable because it is too massive to
be stable.
Some smaller atoms are unstable because the
ratio of protons to neutrons is not ideal – there
may be too many or too few neutrons.
Technetium and promethium are the only
smaller atoms with no stable isotopes.
Does samarium have a stable isotope?
Does berkelium have a stable isotope?
Radioactive Decay
This graph shows the
ratio of neutrons
(going across the x
axis) to protons
(going up the y axis).
The red line shows
where protons and
neutrons are equal in
number. The green
region has stable
isotopes. Isotopes
whose protons and
neutron ratio put them
in the yellow region
are unstable.
Radioactive Decay
Now for the mechanics of radioactive
decay (YAY!)
Radioactive decay comes in three
varieties: alpha, beta, and gamma.
Alpha Decay
Alpha decay, symbolized
α (Greek letter alpha) is
the least penetrating form
of radiation.
It consists of a relatively
large particle consisting
of two protons and two
neutrons – the same as a
helium nucleus.
Because the particle is
large, it can be stopped
by simple means – a
sheet of paper stops it.
http://library.thinkquest.org/28383/grafika/1/arozpad-alfa.gif
Alpha Decay
In alpha decay, the nucleus gives off this particle, called
an alpha particle.
In the diagram below, a uranium atom emits an alpha
particle, consisting of two protons and two neutrons.
Since it now has two fewer protons, the atom is no
longer uranium. What has two fewer protons than
uranium?
Thorium does! Notice also that the mass number has
also gone down by 4; two protons and two neutrons
have left.
http://t1.gstatic.com/images?q=tbn:2a496FevcUXDrM:http://www.btinternet.com/~j.doyle/SR/Emc2/decay_a-decay.gif
Alpha Decay
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http://www.cna.ca/curriculum/cna_radiation/images/alpha3.jpg
This is the equation representing the alpha decay of astatine-211.
Notice that in this example, the alpha particle is represented differently
than in the previous example.
What new daughter atom was created by the emission of the alpha
particle from the At-211?
Why did the atomic number go down by 2?
Why did the mass go down by 4?
Alpha Decay
In alpha decay, the atomic number always
goes down by two, because two protons
have been emitted.
In alpha decay, the mass number always
goes down by four, because four particles
in total have left the nucleus.
Alpha Decay
What is the equation for the alpha decay of curium-247?
Start by writing the symbol of curium.
Put the mass number to the left, up high.
Put the atomic number to the left, down low.
Draw the arrow to the right of the symbol.
Write the symbol for an alpha symbol.
Add a plus sign after that.
Write the atomic number of the new daughter atom.
Write the mass of that new number right above it.
Use your PT of E to find what the new element is and
write its symbol next to the mass and atomic numbers.
Alpha Decay
Beta Decay
Beta decay, symbolized 
(Greek letter beta)
involves a much smaller
particle.
The beta particle is an
electron. As such, it has
much more energy,
speed, and penetrating
power than an alpha
particle. It takes a sheet
of aluminum to stop a
beta particle.
What’s the difference
between the before and after
atoms?
One neutron has turned into
a proton! It does this by
emitting an electron, a beta
particle.
Beta Decay
In beta decay, a neutron turns into a proton and
an electron.
The electron is emitted as beta radiation, while
the proton stays in the nucleus.
That means that the nucleus has one more
proton than before, changing the identity of the
atom.
It has one fewer neutrons, but that does not
matter much, except to make it more stable.
In beta radiation, the atomic number goes up by
one, but the mass does not change.
Beta Radiation
To write the equation for beta radiation, we start
with the complete symbol for our starting atom,
which in this case is cobalt-61.
We then add the arrow and the symbol for the
beta particle.
What will the atomic number for the new
daughter atom be?
Beta Radiation
The atomic number has gone from 27 to
28, because a new proton was created!
What will the new mass be?
Hmm… A neutron became a proton, but
that does not change the mass.
Beta Radiation
The new mass is the same as the old mass.
Often, a radioactive atom will do both alpha and
beta radiation, one right after the other. That
makes the atomic number go down two, then up
one.
Alpha and Beta Radiation
Here’s an example of both happening
sequentially.
In truth, gamma radiation would also be
emitted.
Gamma Radiation
Gamma radiation does not have a particle at all;
it’s a high-energy wave.
Gamma radiation is very penetrating. To stop it
requires a thick concrete barrier.
Gamma Radiation
Gamma radiation, symbolized γ (Greek
letter gamma) seldom happens on its own.
It is a by-product of alpha and beta
radiation.
Because gamma radiation is a wave and
not a particle, it does not change the
composition of the nucleus.