Energy and Radioactivity
Time to Wrap This Energy Unit Up!



We have just finished discussing energy and its relation to
atomic structure as well as its relation to an atom’s ability
to bond
What’s interesting is much of our understanding of
atomic structure came from studies of radioactive
elements
So, what is radioactivity?

To begin answering this question, we must first revisit the
structure of the nucleus of the atom
The Nucleus



Remember that the nucleus is comprised of the two nucleons,
protons and neutrons
The number of protons is the atomic number
The number of protons and neutrons is the mass number


This is because the number of protons and neutrons together
effectively accounts for entire mass of the atom
To help us understand nuclear structure, let’s use some magnets!
Time for the Magnet Demonstration!
Answer Question #1 on Page ______ with Your Neighbor!
The Magnet Demonstration and Its
Significance to the Nucleus
• Any element with more than one proton (all but hydrogen) will have repulsions between the protons in
the nucleus
• The magnets represented the protons in the demonstration
• A strong nuclear force helps keep the nucleus from flying apart
• This force was represented as the tape in the magnet demonstration
• The strong nuclear force It is essentially the “glue” that holds the protons and neutrons together
• It is stronger that the repulsive forces that exist between two protons or other like-charges
• However, it has a very short range – it can’t even reach from one end of a fair-sized atomic nucleus
to the other
• If the balance between the electrical force and strong nuclear force gets upset and the electrical force
wins, some part of the nucleus can go shooting off!
• Radioactivity!
Isotopes and Radioactivity

So, what helps balance the electrical force and the
strong nuclear force interactions?

Come to find out, neutrons play a key role stabilizing
the nucleus


Represented as the marble in the magnet demonstration
The ratio of neutrons to protons is key to determine
the stability of a nucleus
Let’s Investigate!
Turn to Page ______ and Answer the Questions with a Partner!
Neutron-Proton Ratios

As you saw in the “Belt of
Stability” graph, it takes a
greater number of
neutrons to stabilize the
nucleus as nuclei get
larger

This is because repulsions
between protons become
much greater as atomic
number increases, and
more neutrons are
required to hold them
together
Neutron-Proton Ratios

For smaller nuclei (Z  20)
stable nuclei have a
neutron-to-proton ratio
close to 1:1
Magic Numbers in the Nucleus

As we just saw, some combinations of protons and
neutrons appear to be more stable than others


Therefore, there must be ways to predict nuclear stability
One concept involves the idea of magic numbers

Magic numbers are natural occurrences in isotopes and are
stable



Proton magic numbers - 2, 8, 20, 28, 40, 50, 82, 114
Neutron magic numbers - 2, 8, 20, 28, 40, 50, 82, 126, 184
The existence of magic numbers indicates the existence of
energy levels in the nucleus, similar to those observed for
electrons outside the nucleus!

Referred to as the “Shell Model of the Nucleus”

The magic numbers correspond to closed shells in nuclei!
Predicting Nuclear Stability

Another way to predict nuclear stability is based on
whether the nucleus contains odd/even number of protons
and neutrons

Nuclei with an even number of protons and neutrons tend to
be more stable than nuclides that have odd numbers of these
nucleons
Radioactivity and Nuclear Transformations

If a nucleus does not follow either one of these patterns,
it is referred to as being unstable or radioactive



Called radioisotopes
Beyond element 82, all nuclei are radioactive because
there is no amount of neutrons that is strong enough to
hold the nucleus together in a stable manner
So, what happens when a nucleus is unstable?


These radioisotopes gain stability by undergoing radioactive decay
Radioactive decay can occur by emitting mass, charge, or
energy

Note, nuclear reactions can not be speeded up, slowed down,
or stopped – they will always occur!
Discovery of Radioactivity

Radioactivity was first discovered by Marie Curie and
Henri Becquerel
Discovery of Radioactivity
1852 - 1908
1867 - 1934
Nuclear Reactions

The chemical properties of the nucleus are
independent of the state of chemical combination of
the atom


In other words, when writing nuclear equations, it makes no
difference if the atom is as an element or a compound
However, mass and charges MUST BE BALANCED!!!
Types of Radioactive Decay – Positron
Emission

This type of decay occurs when an atom doesn’t have
enough neutrons to be stable

A proton turns into a neutron and loses charge


As a result, a positron (a particle that’s just like an electron except that
it has opposite electric charge) is emitted
The atomic number decreases by one, but the mass number
remains the same
Types of Radioactive Decay – Electron
Capture

This type of decay is viewed as an electron being pulled
into the nucleus from the innermost electron orbit
(energy level)

As a result, a proton is transformed into a neutron!
1
1


p
+
0
−1
e

1
0
n
Competes with positron emission
This decay is a mode of decreasing atomic number while
maintaining mass number
Types of Radioactive Decay – Alpha Decay

This type of decay involves the loss of an α-particle

An α-particle is essentially a particle containing 2 protons, 2 neutrons,
and a +2 charge

A charged helium atom!
4
2

Why helium?

Helium is held together exceptionally tight




2+
He
Makes alpha particles the easiest type of “clump” to spit out
Alpha particles are attracted to the negative plate when passed
through an electric field
In this type of decay, the mass number decreases by four and the
atomic number decreases by two
This type of emission is low energy, but most massive!
Example - Alpha Decay of Uranium

Uranium decays into thorium and an alpha particle

The equation for the nuclear decay is balanced, with the sum
of mass and atomic numbers being equal on both sides of the
equation
Sum of atomic numbers: 92 = 90 + 2
 Sum of mass numbers: 238 = 234 + 4

Types of Radioactive Decay – Beta Radiation



This type of decay is essentially a charge decay
The emitted particle is a result from the breaking up of a neutron
When a neutron decays, it produces a proton (stays in nucleus) and a
beta particle (ejected electron)



The beta particle is essentially a fast-moving electron
The beta particle is higher energy than alpha particles
It is attracted to the positive plate when passed through an electric
field
beta decay of uranium
Types of Radioactive Decay – Gamma
Emission




After alpha or beta decay, a nucleus is often left in an excited
state (with some extra energy)
It then “calms down” by releasing this energy in the form of a
high energy photon
 Called a -ray
The equation for the nuclear decay is balanced, with the sum of
mass and atomic numbers being equal on both sides of the
equation
This type of decay is not affected by an electric field
Penetration of Various Radioactive Decays
The Belt of Stability and Predicting
Type of Decay
 The shaded region in
the figure shows
what nuclides would
be stable, the socalled belt of stability
Stable Nuclei


Nuclei above this belt
have too many
neutrons
They tend to decay by
emitting beta particles

This decreases the
neutrons and increases
the number of protons
Stable Nuclei


Nuclei below the belt have
too many protons
They tend to become
more stable by positron
emission or electron
capture


Both lower the number of
protons!
Positron emission is more
common in lighter nuclei
whereas electron capture
is more common for
heavier nuclei
Stable Nuclei


As stated earlier, there are no stable nuclei with an
atomic number greater than 83
These nuclei tend to decay by alpha emission
Radioactive Series


Large radioactive nuclei
cannot stabilize by
undergoing only one
nuclear transformation
They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead)
Energy Changes in Nuclear Reactions

The nuclear binding energy is an energy required to
break up a nucleus into its components


In essence, it is a quantitative measure of the nuclear stability
Remember, the nucleus contains mainly two particles –
protons and neutrons


Thus, the mass of the nucleus primarily comes from the
masses of protons and neutrons!
But, experiments have known that the sum of the masses of
protons and neutrons is always greater than experimentally
determined nuclear mass

Wait…what?!
The Nucleus and Nuclear Binding Energy

To answer this question, we must look at the way nature
creates the nucleus


Protons and neutrons are bound together and placed in a tiny
space called the nucleus
In order to bind protons and neutrons together, some energy
is needed, which is taken out of the masses of protons and
neutrons


Called the mass defect
Hence, the concept of nuclear binding energy is based on
Einstein’s famous equation, to which energy and mass are
interconvertible:
E = mc2
Einstein and Nuclear Binding Energy

A more useful version of the equation is:
∆E = ∆mc2
where:
∆E = the binding energy
∆m = mass difference between the nucleus and the separate
nucleons
 We can use this relationship to determine how much
energy is produced by a decrease in mass
Binding Energy

We can calculate the binding energy per nucleon for all of the
stable isotopes to compare their relative stabilities.
We end up with the following plot.:
Relative binding Energy
per nucleon

Fe
56
Most stable
Mass number
Binding Energy

As the total number of
nucleons increases, we reach a
point where the binding
energy is at a maximum


Hence the reason why higher
mass nucleons are less stable!
This is why we can obtain
energy from both fission and
fusion and why alpha emission
is common for heavier
isotopes!
Max. Binding Energy
Fusion
Fission
Fe
Getting Power from Nuclear Reactions

Nuclear fusion - the direct combination of 2 radioactive
atoms to produce an atom of larger masses



Requires high temperature to speed up molecules
Nuclei must be forced extremely close together
This is how the Sun gets energy:
4 1H

1
2He
4
+ 2 1e
0
4 hydrogen
1 helium + 2 positrons
atoms
atom
Nuclear fission - splits an atom


Some mass is converted into energy
Nuclear reactors are designed to control the fission process via
a critical chain reaction
What are Chain Reactions?
 Critical Reaction
 When just enough fissions occur to keep the
chain reaction going
(neutrons formed = neutrons used)
 Creates useful nuclear power
• Supercritical Reaction
• When excess neutrons are produced and the
rate of fission keeps increasing at an
uncontrolled rate
• Creates nuclear bombs
Nuclear Reactions
Fission and Fusion
Inside a Nuclear Reactor
Inside a Nuclear Reactor
Inside Chernobyl
The Rates of Nuclear Reactions


Although we can predict the methods by which an
isotope may undergo radioactive decay to become more
stable, we can not predict how quickly these changes will
occur
This brings us to our next unit of study – chemical
kinetics

What is the rate at which a chemical reaction will occur?