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.