Download Alpha Decay Alpha decay can most simply be described like this: 1

Alpha Decay
Alpha decay can most simply be described like this:
1. The nucleus of an atom splits into two parts.
2. One of these parts (the alpha particle) goes zooming off into space.
3. The nucleus left behind has its atomic number reduced by 2 and its mass number reduced by 4 (that
is, by 2 protons and 2 neutrons).
There are other points, but the three above are enough for this class. Here is a typical alpha decay
equation:
ଶଷ଼
ଶଷସ
ସ
ଽଶ →ଽ଴ ℎ + ଶ Notice several things about it:
1. The atom on the left side is the one that splits into two pieces.
2. One of the two atoms on the right is ALWAYS an alpha particle.
3. The other atom on the right ALWAYS goes down by two in the atomic number and four in the mass
number.
ଶସଵ
ଽହ ସ
→ଶଷ଻
ଽଷ + ଶ Check it and compare the three points to the example. Keep in mind that this equation shows the left-hand
side splitting into the two pieces shown on the right-hand side.
Beta Decay
Beta decay is somewhat more complex than alpha decay is. These points present a simplified view of
what beta decay actually is:
1. A neutron inside the nucleus of an atom breaks down, changing into a proton.
2. It emits an electron and an anti-neutrino (more on this later) which go zooming off into space.
3. The atomic number goes UP by one and mass number remains unchanged.
Here is an example of a beta decay equation:
ଷ
ଵ
→ଷଶ + ଴ିଵ + ଴଴ ̅
Some points to be made about the equation:
1. The nuclide that decays is the one on the left-hand side of the equation.
2. The order of the nuclides on the right-hand side can be in any order.
3. The way it is written above is the usual way.
4. The mass number and atomic number of the antineutrino are zero and the bar above the symbol
indicates it is an anti-particle.
5. The neutrino symbol is the Greek letter "nu."
Here is another example of a beta decay equation:
ଵଷ଻
ହହ ଴
଴
→ଵଷ଺
ହ଺ + ିଵ + ଴ ̅
Notice that all the atomic numbers on both sides ADD UP TO THE SAME VALUE and the same for the
mass numbers.
Positron Decay
Positron decay is like a mirror image of beta decay. These points present a simplified view of what
positron decay actually is:
1. Something inside the nucleus of an atom breaks down, which causes a proton to become a neutron.
2. It emits a positron and a neutrino which go zooming off into space.
3. The atomic number goes DOWN by one and mass number remains unchanged.
Here is an example of a positron decay equation:
ଵଵ
଺ ଴
଴
→ଵଵ
ହ + ାଵ + ଴ Some points to be made about the equation:
1. The nuclide that decays is the one on the left-hand side of the equation.
2. The nuclides on the right-hand side can be in any order.
3. The way it is written above is the usual way.
4. The mass number and atomic number of the neutrino are zero.
5. The neutrino symbol is the Greek letter "nu."
Here is another example of a positron decay equation:
ଵଶହ
ହ଻ ଴
଴
→ଵଶହ
ହ଺ + ାଵ + ଴ Notice that all the atomic numbers on both sides ADD UP TO THE SAME VALUE and the same for the
mass numbers.
Electron Capture
Electron capture is not like any other decay – alpha, beta, or position. All other decays shoot something
out of the nucleus. In electron capture, something ENTERS the nucleus. These points present a
simplified view of what electron capture is:
1. An electron from the closest energy level falls into the nucleus, which causes a proton to become a
neutron.
2. A neutrino is emitted from the nucleus.
3. Another electron falls into the empty energy level and so on causing a cascade of electrons falling.
One free electron, moving about in space, falls into the outermost empty level. (Incidently, this
cascade of electrons falling creates a characteristic cascade of emission lines. This is the fingerprint
of electron capture.)
4. The atomic number goes DOWN by one and mass number remains unchanged.
Here is an example of a electron capture equation:
଻
ସ + ଴ିଵ →଻ଷ + ଴଴ Some points to be made about the equation:
1. The nuclide that decays is the one on the left-hand side of the equation.
2. The electron must also be written on the left-hand side. A neutrino is involved, it is ejected from the
nucleus where the electron reacts, so it is written on the right-hand side.
3. The way it is written above is the usual way.
Here is another example of a electron capture equation:
ଶଶ
ଵଵ ଶଶ
+ ଴ିଵ →ଵ଴
+ ଴଴ Notice that all the atomic numbers on both sides ADD UP TO THE SAME VALUE and the same for the
mass numbers.
Gamma
Gamma radiation ( ) is the emission of high energy photons in the range of 10-10 m, with no matter
associated with it. Gamma rays are normally the by-products of other alpha or beta emissions. Gamma
rays have no effect on either mass or charge, gamma rays only stabilize the nucleus by releasing some of
the excess energy. Gamma rays are the most damaging rays being able to penetrate meter thick lead.
High energy gamma rays are known as cosmic rays are capable of penetrating kilometers of dense
material.
Half-Life
When unstable nuclei undergo radioactive decay, their decay rate is not steady. Instead, they have a halflife (), which is the amount of time required for half of the reactant to disappear. For example, Iodine-131
is unstable and under beta particle emission, it becomes Xenon-131. It has a half life of roughly 8 days.
So if we had a 4 gram sample of Iodine-131, in 8 days, there would be 2 grams left. In another 8 days,
there would be 1 gram.
Xenon-131
Xenon-131
Xenon-131
Xenon-131
Iodine-131
Iodine-131
Iodine-131
Iodine-131
Iodine-131
0 days
8 days
16 days
24 days
32 days
How to Solve
(starting amount) x (1/2)number of half-lives = ending amount
number of half-lives can be expressed as (total time elasped ÷ length of half-life)
Problems
Write the alpha decay for the following: Pu-239, Gd-149, Au-185, Th-234
Write the beta decay for the following: C-14, I-131, He-6, Np-239
Write the positron decay for the following: Ba-124, Br-75, P-30, B-8
Write the electron capture for the following: Rb-82, W-178, Md-257, As-73
The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be
left after 7.2 minutes has elapsed?
6. How long will it take the 40.0 grams sample of I-131 (half-life = 8.040 days) to decay to 1/128 its
original mass?
7. 160.0 grams of an isotope with a half-life of 36.0 hours is present at time zero. How much time will
elapse before 80.0 grams remains? Before 5.00 grams remains?
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