The Nucleus and Radioactivity
Radioactivity: Spontaneous changes in the nucleus that emit
energy as radiation (particles or rays)
Nuclei contain protons and neutrons; some combinations of these
particles are unstable
Examples of Radioactive Nuclei Include:
Uranium, Plutonium
Hydrogen-3
Potassium-38
Radioactive Decay: Emission of radiation produced by unstable
nuclei changing to a more stable state
Types of Radiation Include:
Alpha rays a: positive charge
Beta rays b: negative charge
Gamma rays g: no charge
a and b rays consist of streams of particles
g rays consist of electromagnetic radiation
positron: an antiparticle of a β particle (their charges are opposite,
but their masses are the same)
•A positron has a +1 charge and is called a “positive electron.”
0
e
+1
•A positron is formed when a proton is converted to a neutron.
positron:
1
p
1
proton
β+ or
1
n
0
neutron
0
e
+
+1
positron
a particle:
contains 2 protons and 2 neutrons
identical to helium nucleus
travel only short distances
b particle:
electrons produced in the nucleus, then emitted
travel greater distances than a particles
g Ray:
High-energy ray similar to an X ray
Travel great distances
Daughter Nuclei: New nuclei that result from unstable
nuclei undergoing radioactive decay
Example: Uranium-238 gives up an a particle, resulting in a
daughter nucleus of a different element, Thorium (Th)
Summary of Radiation Types
Alpha Decay
• When a radioactive nucleus emits an alpha particle, a new
nucleus results.
• The mass number of the new nucleus is 4 less than that of the
initial nucleus.
• The atomic number is decreased by 2.
Nuclear Reactions: Alpha Emission
Alpha emission is the decay of a nucleus by emitting
an a particle.
In a balanced nuclear equation, the sum of the mass numbers
and the sum of the atomic numbers for the nuclei of the
reactant and the products must be equal.
251Cf
98
241
95
247Cm
96
Am
4
2
+
4He
2
He
+
237
93
Np
Write an equation for the alpha decay of Rn-222.
222Rn
new nucleus +
86
4He
2
Mass number: 222 – 4 = 218
86 – 2
Atomic number:
Symbol of element 84
222Rn
86
218Po
84
= 84
= Po
+
4He
2
Beta Decay
 The unstable nucleus converts a neutron into a proton (emitting an
electron from the nucleus)
 The mass number of the new nucleus remains the same
 The atomic number of the new nucleus increases by 1
1n
0
0e
-1
+ 1H
1
Nuclear Reactions: Beta Emission
Beta emission is the decay of a nucleus by emitting a β
particle; 1 neutron is lost and 1 proton is gained.
Example: Potassium - 42 is a beta emitter.
42K
new nucleus +
19
-1
Mass number :
(same)
Atomic number:
19
42Ca
20
= 42
19 + 1 A
Symbol of element 20 = Ca
42K
0e
+
0e
-1
= 20
Learning Check
Write the nuclear equation for the
beta decay of Co-60.
60Co
27
Solution
Write the nuclear equation for the beta
decay of Co-60.
60Co
60Ni
27
28
+ 0e
1
Nuclear Reactions: Positron Emission
Positron emission is the decay of a nucleus by emitting a
positron, β+; 1 proton is lost and 1 neutron is gained.
16
Gamma g Radiation
• Gamma radiation is energy emitted from an unstable nucleus
indicated by m.
• In a nuclear equation for gamma emission, the mass number and
the atomic number are the same.
99mTc
43
99Tc
43
+ g
Summary of Radiation
Some radioactive isotopes are more stable than others, and therefore
decay more slowly
Half-Life: Time required for half of the unstable nuclei in a sample
to decay
Example: A Potassium-38 sample weighs 100 grams. 8 minutes
later, the sample is weighed again and found to weigh 50 g. The
half-life of potassium-38 is 8 minutes
Note: The half-life of a radioactive isotope is a property of a given
isotope and is independent of the amount of sample, temperature,
and pressure.
Half-Lives Vary Dramatically Between Elements
Half-Life Calculations
After one half-life, 40 mg of a radioisotope will
decay to 20 mg. After two half-lives, 10 mg of
radioisotope remain.
40 mg x 1 x 1 = 10 mg
2
2
Initial
40 mg
1 half-life
20 mg
2 half-lives
10 mg
Practice:
If the half-life of iodine-131 is 8.0 days, how much of a
100. mg sample remains after 32 days?
Determine how many half-lives occur in the
given amount of time.
32 days
x
1 half-life
8.0 days
=
4.0 half-lives
For each half-life, multiply the initial mass by one-half to obtain
the final mass:
100. mg
x
1
2
x
1
2
x
1
2
x
1
2
= 6.25 mg
final mass
initial mass
The mass is halved four times.
Learning Check
The half life of I-123 is 13 hr. How
much of a 64 mg sample of I-123 is
left after 26 hours?
Solution
Half life = 13 hrs
Number of half lives
Amount remaining
= 2
= 64 mg x 1 x 1 = 16 mg
2
2
13 hrs
64 mg
13 hrs
32 mg
16 mg
Radiation and Health
Free Radicals: Very reactive compounds that can cause
mutations, cancer; usually caused by long-term exposure to lowlevel radiation
Radiation Sickness: Illness and symptoms caused by shortterm exposure to intense radiation
Uses of Radioisotopes
Medical: diagnosing and disease (cancer, thyroid, brain scans)
Common Imaging Techniques
PET Scans (Positron Emission Tomography): gamma rays create a
3D image of organs, used to analyze blood flow, metabolic activity and
brain function
CT (Computed Tomography): X-rays are used to create series of
images of the brain, identifying brain damage and hemorrhaging
MRI (Magnetic Resonance Imaging): H protons in magnetic field are
used to create color images of soft tissue
Health/Agriculture: food irradiation
Radioactive dating: determine age of fossils
• Nuclear Power Plants: Alternative energy source
Units of Radiation
Curie (Ci): number of disintegrations per second per gram of radium;
3.7 x 1010 disintegrations per second
Rad (Radiation Absorbed Dose): amount of material able to deliver
2.4x10-3 cal of energy to 1 kg of tissue
Rem (Radiation Equivalent in humans): amount of biological damage
caused by different types of radiation
In 1934 Radioactivity was Artificially Induced for the first time!!
High-energy particles (such as neutrons) can create unstable nuclei that
then undergo radioactive decay (Cyclotrons and Linear Accelerators)
Nuclear Fission: Process in which large nuclei split into smaller nuclei
when bombarded with neutrons, releasing large amounts of energy
Example: When a neutron bombards U-235, an unstable nucleus of
U-236 forms smaller nuclei such as Kr-91 and Ba-142.
Chain Reaction: Nuclear reaction in which the products of a reaction
cause that reaction to occur repeatedly
Nuclear Fusion: Process in which small nuclei combine (fuse) to
form larger nuclei
Example: Hydrogen nuclei combine to form Helium nuclei