Chem 112 Class Guide: NUCLEAR DECAY
Chapter 21, Sections 1, 2 and 3 Learning Goals:
Upon completion of Chapter 19, Sections 1-4, you should be able to determine the following:
Identify α, β, and ϒ particles
Predict the products of α, β, ϒ, positron decay or electron capture reaction
Chapter reading Guide: Chapter 21, Sections 1, 2 and 3
Section 1: Radioactivity
Read Chapter 21.1
Before we begin, it might be a good idea to review Chapter 2 Section 3 about isotopes.
Remember that 146C has a mass number of 14 (neutrons plus protons) and an atomic
number of 6 (number of protons). You should be able to determine that this isotope of
carbon has 8 neutrons. Also, we could write this as carbon-14 instead of 146C .
A nuclide is a nucleus containing a specific number of protons and neutrons. This is another
way of saying isotope.
While the vast majority of nuclides are stable, radionuclides are unstable and spontaneously
emit particles and/or electromagnetic radiation. The emission of particles is called nuclear
decay.
There are 5 types of nuclear decay:
1.) Alpha Decay
Occurs when a radionuclide emits an alpha particle:
U
238
92
Th  24 (Note that the mass number and atomic numbers on both sides of
234
90
the equation are equal, or 238 = 234 + 4 and 92 = 90 + 2.)
2.) Beta Decay
Occurs when a radionuclide emits a beta particle:
131
53
0
I  131
54Th  1  (Again, the mass numbers and atomic numbers on both sides of the
equation are equal. This will be true for all types of decay.)
3.) Gamma decay
Occurs when a radionuclide emits a gamma particle:
U  00
U
238
92
238
92
4.) Positron Emission
This decay occurs when a radionuclide emits a positron. A positron is like an electron,
but it has a positive charge instead of a negative charge.
C  115 B  10 e
11
6
5.) Electron Capture
Not technically a decay, as the reactant captures an electron
81
37
Rb  10 e  3681Kr
Table 21.3 summarizes these reactions.
Example:
What particle is produced when plutonium-242 decays to uranium-238?
We need the atomic numbers and the mass numbers to equal on both sides of the arrow, so
we need all the information about the nuclides to get a rough idea of where to begin (use
the periodic table for this):
242
94
Pu 
238
92
U
The mass number of Pu is 4 greater than that of U and the atomic number of Pu is 2 greater
than U. An alpha particle has a mass number of 4 and an atomic number of 2, so we need
an alpha particle on the products side of the equation to balance:
242
94
Pu 
U  24
238
92
Try practice exercises 21.1 and 21.2
Section 2: Patterns of Nuclear Stability
Read Chapter 21.2
It is possible to predict the type of the type of decay a nuclide will go through by looking at
the ratio of neutrons to protons and looking at the “belt of stability” (figure 21.2)
Nuclei with a higher than “normal” neutron-to-proton ratio will likely decay via beta
emission.
Nuclei with a lower than “normal” neutron-to-proton ratio will likely decay via positron
emission OR electron capture
Nuclei with 84 or more protons are likely to decay via alpha decay
Example:
Predict the type of radioactive decay iodine-120 undergoes.
To do this, we need to calculate the neutron/proton ratio.
neutrons (120 – 53). The ratio is:
120
53
I has 53 protons and 67
67
 1.26 . This ratio is higher than normal (>1.00) so will
53
likely undergo beta emission:
120
53
I  10 e  120
52Te
Remember that when the number of protons changes (goes from 53 to 52), the identity of
the element changes.
Nuclides that have an even number of neutrons, protons or both are more likely to be stable
than nuclides with odd numbers of protons and/or neutrons.
Also, elements with full shells are more likely to be stable than nuclides with other numbers
of protons and/or neutrons (full shells are: 2, 8, 20, 28, 50, 82 and 126).
Try practice exercise 21.3
Section 3: Nuclear Transmutations
Read Chapter 21.3
A nucleus can change if it is struck by a neutron or another nucleus.
Example:
Write a balanced equation when
14
7
N reacts with an alpha particle to produce 116C and a
proton.
14
7
N  24  116 C  11H Remember that the atomic numbers and the mass numbers have to
equal on both sides of the arrow!
We can write the reaction above another way:
14
7
N  , p  116 C . The nucleus on the left is
the reactant ( 147 N ), the item on the left inside parentheses is the particle that struck the
reacting nuclei (alpha particle), the item on the right inside parentheses is the particle that is
released (p, or proton), and the nuclei on the right is the nuclei that is formed in the
reaction.
Some of the abbreviations you will see:
Abbreviation Particle
4
α

2
β
0
1
γ
0
0
n
1
0
p
1
1


n
H
Try Practice exercise 21.5
Learning Resources
Chapter Learning Goals
Chapter 20, Sections 1, 2 and 3 Learning Goals
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Complete to the pre class assignment
(http://berks.psu.edu/clt/chem112/NuclearDecay_HW.docx)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission:
Nuclear Decay”
End of Chapter Problems:
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
help from your instructor, your mentor or the learning center
Chapter 21: 7, 9, 13, 17, 27