In the late years of the 19th century scientists
were interested in experiments about the rays
that certain elements were emitting. After
various experiments they found that there was
a different ray then x rays which could go
further and deeper and could pass thicker
obstacles through experiments. Soon a
scientist called Becquerel discovered that
various substances containing uranium
admixtures emitted such radiation. This new
phenomenon was to be called radioactivity.
Rutherford and others began studying the
nature of the rays emitted in radioactivity
about 1898. They found that the rays could be
classified into three distinct types according to
their penetrating power. One type of the
radiation could barely penetrate a piece of
paper. The second type could pass through as
much as 3mm of aluminum. The third was
extremely penetrating: it could pass trough
several centimeters of lead and still be
detected on the other side. They named these
three types of radiation alpha (α) , beta (β)
and gamma (γ), respectively, after the first
three letters of the Greek alphabet.
The alpha decay occurs when the nucleus
of an element emits an alpha particle which
is two protons and two neutrons. This is a
simple helium atom with no electron
present. An example of this is a uranium238 atom decaying into into a thorium-234
atom and an alpha particle (helium-4
nucleus, i.e. 2 protons and 2 neutrons).
1
1p
+
7
3 Li
proton + lithium
4
Be + 2 2 He
beryllium + 2 alpha
particles
(http://library.thinkquest.org/3659/nucreact/transmutation.html)
• As you may wonder; why does alpha decay
occur?
• Alpha decay occurs because the strong nuclear
force is unable to hold very large nuclei
together. Because the nuclear force is a shortrange force, it acts only between neighboring
nucleons. For every nuclei, the large atomic
number means the repulsive force becomes
very large (Coulomb's law) and it acts between
all protons. The strong nuclear force, since it
acts only between neighboring nucleons, is
overpowered and is unable to hold the nucleus
together.
• But if there is a
repelling force between
the protons why not all
the protons leave the
nucleus? This should
be explained by
another force holding
them together called as
Strong Nuclear Force.
Indeed, the question arises as how a
nucleus stays together at all in the view of
the fact that the electric force between
protons would tend to break it apart since
stable nuclei stay together,it is clear that
another force must be acting.
Because this force is stronger than the
electric force it is called the strong nuclear
force. The strong nuclear force is an attractive
force that acts between all nucleons-protons
and neutrons - alike .
Thus,protons attract each other via the
nuclear force and at the same time they repel
each other via the lectric force.
Neutrons, since they are electrically
neutral, only attract other neutrons or protons
via the nuclear force.
This way, the nucleus does not break apart
and stay together.
• Alpha decay is an exothermic process. As the
nucleus becomes more stable, it liberates a net
amount of energy due to this. Therefore, the
tendency for minimum enthalpy is fulfilled.
Also, the decay’s entropy can be said to be
positive; the places the particles can exist
increase.
• Under these conditions, it can be said that
Alpha decay is always spontaneous.
Energy Changes in Alpha Decay
This type of decay occurs naturally in uranium and
is an example of "spontaneous decay".
The uranium atom doesn't just break apart. As it
decays, each of the two resulting elements (the
thorium and α-particle) fly apart at high speed. In
other words they both have kinetic energy.
It is possible to measure the mass of the original
uranium atom and the masses of the two resultant
particles. This is done by measuring the momentum of
each particle as it strikes a sensor. When these
measurements are taken it is found that the total
mass of the two smaller particles is less than the
mass of the original uranium particle. Some mass must
have been turned into (mostly kinetic) energy.
So as we have said the mass and energy is not
conserved during a radiation.
• Write a balanced equation for the following
process:
•
237
93
Answer:
Np (Np turns into Pa)
237
Np
93
233
91
Pa +
4
2 He
• Write a balanced equation for the process
that is described below:
• Uranium (235); which is used in atomic
bombs, decays initially by alpha particle
production.
235
U
92
231
90 Th
+ 4 He
2
When an alpha decay occcurs;
1) Mass number decreases by 2
2) It’s chemical properties change
3) Neutron number decreases by 2
Which of the following above statements
are correct?
A) Only 1
C) 2 and 3
E) 1,2 and 3
B)1 and 2
D) 1 and 3
Answer : C
226
When the atom 88Ra makes 3 alpha
radiation what would be the neutrons
number of the newly formed element?
A) 214
D) 132
B) 164
E) 82
C) 142
Answer:
226 Ra
88
3 4 He+
2
3 x 4= 12
226-12= 214
3x2=6
88-6=82
?X
?
214 – 82 = 132 So the answer is D.
214
82X
QUESTION THAT WAS NOT
ANSWERED...
Why does in alpha decay a simple
helium atom is emitted instead of
saperate individual particles??
•There was a lot of verbal and written mistakes.
•I did not talked deeply about the subjects such
as energy change during alpha radiation which
can last for a very long time.
•I could not answer some of the questions came
from the class.
•The subjects were presented in a very easy level.
•It was easy to understand the topics and contents
but scientifically the presentation is not enough.
•Many subjects that should have been placed in
the presentation was skipped so it was not
sufficent.