Unstable Nuclei
and Radioactive
Decay
CHAPTER 4
SECTION 4
Main Idea
Unstable atoms
emit radiation
to gain stability!!
Essential Questions & Vocabulary
What is the relationship between unstable nuclei and
radioactive decay?
How are alpha, beta, and gamma radiation characterized in
terms of mass and charge?
Vocabulary
Radioactivity
Radiation
Nuclear reaction
Radioactive decay
Alpha radiation
Alpha particle
Nuclear equation
Beta radiation
Beta particle
Gamma ray
New Terms for Nuclear Chemistry
Atom = nuclide
Protons and Neutrons = nucleon
The Nucleus
The nucleus is composed of nucleons
Protons & Neutrons
Nucleons are bound together by the Strong Force.
Nuclear Reactions vs. Chemical Reactions
Chemical Reactions changes involving
electrons.
Nuclear Reactions –
changes involving nucleus.
Radioactivity
In the late 1890s, scientists noticed that some substances spontaneously
emitted radiation in a process they named radioactivity.
Radiation – the rays and particles emitted by the radioactive matter.
Nuclear Reaction – involves a change to the atomic nucleus
Results in new atoms
Radioactive atoms emit radiation because their nuclei are unstable.
Unstable systems gain stability by losing energy.
Radioactive Decay
Spontaneous process in which unstable nuclei lose energy by
emitting radiation
Types of Radiation
Alpha Radiation
Beta Radiation
Gamma Radiation
Alpha Radiation (α)
Alpha Particle – Helium nucleus with 2 protons and 2 neutrons.
238
92
U
Th He
234
90
4
2
• Mass – 4 amu
• Charge - +2
parent
nuclide
daughter
nuclide
alpha
particle
Numbers must balance!!
Alpha Radiation (α)
Alpha particles have the same composition as a helium nucleus—two protons and two neutrons.
Because of the protons, alpha particles have a 2+ charge.
Alpha radiation consists of a stream of particles.
Alpha radiation is not very penetrating—a single sheet of paper will stop an alpha particle.
Beta Radiation (β or
0
−𝟏𝑒)
Beta Particle – fast moving electron with a -1 charge.
131
53
parent
nuclide
I
131
54
Xe e
daughter
nuclide
0
-1
Beta
particle
Beta Radiation (β or
0
−𝟏𝑒)
Beta particles are very fast-moving electrons emitted when a neutron is converted to a proton.
Beta particles have insignificant mass and a 1– charge.
Beta radiation is a stream of fast moving particles with greater penetrating power—a thin sheet of foil will
stop them.
Transmutation
The conversion of an atom of one element to an atom of
another element.
Gamma Radiation (γ)
Gamma Ray – High energy radiation that possesses no mass and no charge.
Usually accompany alpha and beta radiation.
Account for most of the energy lost during radioactive decays.
238
92
U
parent
nuclide
Th
234
90
daughter
nuclide
4
2
He
alpha
particle
Gamma
ray
Gamma Radiation (γ)
Gamma rays are high-energy electromagnetic radiation.
Gamma rays have no mass or charge.
Gamma rays almost always accompany alpha and beta radiation.
The ability of radiation to pass through matter is called its penetrating
power.
Gamma rays are highly penetrating because they have no charge and no
mass.
Nuclear Reactions
In a nuclear reaction, the total of the atomic numbers
and the total of the mass numbers must be equal on
both sides of the equation.
Example:
9
4
Be +
4
2
a
12
6
C +
1
0
n
Nuclear Reactions - Practice
238
92
U
131
53
38
19
106
47
I
K
Ag
Th
234
90
0
-1
He
Xe
0
-1
Ar
0
1
131
54
38
18
4
2
e
106
46
e
e
Pd
Nuclear Stability
All nucleons remain in the dense nucleus because
of the strong nuclear force.
The strong nuclear force acts on subatomic
particles that are extremely close together and
overcomes the electrostatic repulsion among
protons.
Nuclear Stability
Primary factor in determining an atom’s stability is its ratio of
neutrons to protons.
Atoms with too many or too few neutrons are unstable and lose
energy through radioactive decay to form stable nucleus.
Unstable atoms emit alpha particles and beta particles which
affect their neutron to proton ratio.
Eventually, radioactive atoms undergo enough radioactive
decay until they form stable, nonradioactive atoms.
Nuclear Stability
The area on the graph within which all stable
nuclei are found is known as the band of stability.
All radioactive nuclei are found outside the band.
The band ends at Pb-208; all elements with
atomic numbers greater than 82 are radioactive.
Beyond Bismuth (#83), the repulsive forces of
protons are so great that no stable nuclide exists.
Stable nuclei tend to have even number of
nucleons
Band of Stability
Why does this happen?
- Explained by the relationship between nuclear force and electrostatic forces
between protons.
Protons in the nucleus repel all other protons through electrostatic
repulsion.
As the number of protons increase, the repulsive electrostatic force
between protons increase faster than the nuclear force
More neutrons are required to increase the nuclear force to stabilize the
nucleus.
Neutron to proton ratio increases gradually to about 1.5:1.
Nuclear Stability – Alpha Decay
In alpha decay, nuclei with more than 82
protons are radioactive and decay
spontaneously.
Both neutrons and protons must be
reduced.
Emitting alpha particles reduces both
neutrons and protons.
Nuclear Stability – Beta Decay
In beta decay, radioisotopes above the band of
stability have too many neutrons to be stable.
Beta decay decreases the number of neutrons in
the nucleus by converting one to a proton and
emitting a beta particle.
Nuclear fusion
Nuclear Fission
Nuclear Fusion
• Splitting of atomic nucleus
• Responsible for power generated at nuclear
power plants.
• Combining of atomic nuclei
• Responsible for the glow and heat from stars.
Radioactive Decay Rates – Half-Life
Radioactive decay rates are measured in half-lives.
A half-life is the time required for one-half of a radioisotope to
decay into its products.
N is the remaining amount.
N0 is the initial amount.
n is the number of half-lives that
have passed.
t is the elapsed time and T is the
duration of the half-life.
Half –Life (Radioactive Decay Rate)
Radioactive Dating
Carbon-dating is used
to measure the age of
artifacts that were once
part of a living organism.
Uses
carbon-14 only.
0
