Chemistr
y
Lecture
No.4
______
B
y
:
Asst.
prof.
Tariq-H-AL-mgheer
RADIOACTIVITY AND
WCLEM
CHEMISTRY
Previously we learned that there are isotopes of almost all the elements. Most
of these isotopes are stable, but some are unstable. The nuclei of unstable isotopes
undergo spontaneous nuclear reactions that cause particles and energy, called
nuclear radiation, to be given off. The emission of these particles and energy by an
isotope is called radioactivity. Only a few isotopes found in nature are radioactive.
The first example of a naturally occurring radioactive substance was discovered by
Henri
Becquerel
in 1896. He found that uranium ore gave off penetrating radiation
that darkened photographic film without exposing the film to light. Since then, other
scientists have found many other radioactive elements. More than 50 naturally
occurring radioactive isotopes are now known. In addition, scientists have been able
to make many radioactive isotopes not found in nature.
The discovery of radioactive isotopes has greatly affected our lives. The
awesome power of nuclear weapons, the promise of abundant energy, radiation
therapy, and contamination of the environment by nuclear waste products are all
results of the properties of the tiny nuclei of these radioactive isotopes.
TYPIES
OF
RADIATION
Early in the twentieth century, it was discovered that na
t
urally occurring
isotopes emit three kinds of radiation. At that time, scientists did not understand
them, so their discoverers named them simply alpha (a) beta
(p)
and gamma
(y).
Since then, scientists have discovered the identities and properties of these types of
radiation
.
Each has characteristic properties that determine how it affects living
s
ystems
Alpha radiation is a stream of particles moving at about one-tenth the speed of
light. Each particles is the nucleus of helium atom that contains two protons and two
neutrons and has a charge of +2. Alpha particles are relatively large and heavy, so
they cannot travel very far without colliding with other particles. As a result these
particles do little damage to internal organs because they cannot penetrate the skin.
However, if a substance that emits alpha particles gets inside the body by being
inhaled or swallowed, the alpha particles can then damage internal organ.
Beta radiation is also a stream of particles, bu
t
the particles are electrons. The
electrons are produced within the nucleu
s
by
the transformation of a neutron into a
proton and an electron. The proton stays in the nucleus and the electron is emitted.
An electron is smaller than a helium nucleus (alpha particle), travels much faster, and
can penetrate the skin to a depth of a few centimeters. Exposure to beta radiation
causes the skin to appear burned. Damage to internal organs occurs when a
substance that emits beta particles gets into the body.
Gamma radiation is not a particle, but a form of energy similar to light waves,
radio waves, or x-rays
.
This radiation has high energy and can penetrate deep within
the body and cause serious damage. Gamma radiat
i
on usually occurs along with
alpha and beta radiation.
Two less common but still important types of nuclear radiation are
neutrons
and positrons. A positron has about the same mass as an electron but has a
positive charge. The properties and symbols of these various forms of radiation are
summarized in
Tablel.
The emission of radiation from radioactive isotopes is also
called ionizing radiation.
Table 1.
Properties of Various Forms of Nuclear Radiation
Type of
Radiation
Composition
Symbol
Mass
(amu)
Electrical
Charge
Approximate
penetration of skin
(cm)
Alpha
Helium nucleus
2
He
4
4
+2
0.01
Beta
Electron
,
-
°
e
1/1837
-1
1
Gamma
Energy
Y
0
0
100
Neutron
Neutron
n
,
0
n
1
1
0
10
Pos
i
tron
Positron
,
1
e
0
1/1846
+1
1
IONIZING RADIATION
The radiation from radioactive isotopes and x-rays can form ions in matter by
knocking electrons off the atoms and molecules in its path. For this reason, it is
called ionizing radiation. The chief effects of radiation on living systems are due to
these
ionization
reactions. Repeated exposure to low levels of radiation seems to
have a number of major effects on health. Among them are cancer (carcinogenic
effects), damage to the fetus, and genetic damage.
It has been known for many years that radiation causes cancer. Skin cancer,
bone cancer,
leukemia,
and other cancers are products of exposure to radiation.
Even at very low levels of exposure, there is danger from radiation. For example,
x
rays used in diagnosis are not completely free of potential harm to a patient. Persons
who administer x-rays must take precautions to avoid exposure, because the effect is
cumulative. Exposure to high levels of radiation kills cells. Use is made of this fact
tc
treat cancer. Cancer cells are exposed to high-energy x-rays or gamma radiation to
destroy these cells or to retard the spread of cancer.
Fetuses and small children are particularly sensitive to radiation. Ionizing
radiation affects them more strongly than adults. The effects of radiation are
widespread. Damage occurs to the brain, eyes, bones, and other organs.
The genetic risk of exposure to radiation is more difficult to determine because
the genetic damage may not be seen for several generations. Genetic damage is
caused by damage to the genes in the nuclei of cells. The damage to the structure of
the gene may cause death or a variety of physical defects in following generations.
Clearly, exposure to radiation is dangerous. But is there any level of exposure
below which radiation has no effect? According to one theory, called the threshold
theory, no damage occurs below a certain level of radiation, called the threshold
value. Opposed to this is the linear theory. According to the linear theory, the risk
ofdamage is proportional to exposure, even down to very low levels of radiation. The
current view is a compromise of these two theories: There is a risk of damage even
at low levels of radiation, but the risk is extremely small.
The dangers of ionizing radiation are compounded by the fact that this
radiation cannot be detected by the human body. We cannot see, feel, or smell
ionizing radiation. Therefore, a person can be exposed to lethal levels of radiation
without knowing it until it is too late. But we do have methods of detecting
ionization
radiation.
DETECTING IONIZING RADIATION
The methods of detecting ionizing radiation all make use of the fact that
radiation disturbs the electronic environment of the atoms and molecules that it
encounters; The three methods of detecting ionizing radiation most frequently used
are the photographic method, use of scintillation counters, and use of the
Geiger
counter,
Becquerel
discovered that uranium ore was radioactive because of its effect
on photographic film. Photographic film and paper shielded from light are exposed by
ionizing radiation. This exposure is detected by developing the film or paper in the
usual way. This fact is used to provide an individual detector for persons working
near sources of radioactivity. Each person wears a badge containing a piece of film.
The film is changed at regular intervals and developed. A certain level of radiation will
cause the film to be exposed, warning the wearer of potential danger.
Scintillation counters are instruments that contain a surface coated with a
special substance that gives off flashes of light when hit by ionizing radiation. The
invisible ionizing radiation strikes the surface and some of its energy is transformed
into visible light. Electronic devices magnify and record these flashes.
most common instrument used to detect and measure ionizing radiation is the
Geiger
counter, shown schematically in Figure 1. The detecting part of the instrument
is a metal tube. It contains a gas, a wire down the center, and a window at one end.
The window is made of a thin material to allow alpha and beta particles to enter. A
large potential difference is maintained between the metal walls of the tube and the
central wire. When ionizing radiation enters the tube, it forms ions. This causes a
pulse of electricity to flow from the wire to the metal walls of the tube. This pulse is
counted by an electronic device that produces either a meter reading or an audible
clicking sound. Small, portable Geiger counters are in common use to detect sources
of radioactivity in the environment. When we detect ionizing radiation, it usually means
that a nuclear reaction has taken place.
NUCLEAR REACTIONS
Alchemists in the Middle Ages dreamed of turning one chemical element into
another. The atomic theory seemed to shatter this dream because the atom assumed
the role of the stable and indivisible unit of matter. However, with the discovery of
radioactivity, scientists soon. realized that atoms do change from one kind to another
when they emit nuclear radiation. This change occurs during a nuclear reaction
whenever the nucleus of an isotope emits alpha or beta particles. When this happens,
the nucleus gains or loses positive charges and its atomic number is changed. A
change in atomic number means that one element has changed to another.
.
HALF-LI
FE
The breakup or decay of the nuclei of a particular radioactive isotope requires a
certain amount of time to occur. Not all nuclei decay at the same time rather; they
decay over a period of time. The time needed for one-half of the original nuclei of an
isotope to decay to other substances is called the half-life of the isotope. The symbol
(t
1/
2
)
is used to indicate hall-life.
Radioactivity and nuclear chemistry :-
Isotopes------
Unstable -------- Undergo spontaneous
nuclear reaction ------- Emission energy and particles.
decay
A-------------------- Alpha +Beta +Gamma +
Neutron
+
Positron.
Ratio of (n/p ) used to limited the stability of elements.
Radioactivity serious :-
1-Thorium serious :-
90
Th
234
--------
91
Pa
234
+
-1
β
0
t
1/2
=1.39 × 10
10
years.
Thorium Protactinium
Bata
234/4 = 58 for alpha radiation .
2-Uranium serious :-
92
U
238
-------------
90
Th
234
+
2
He
4
t
1/2
=4.5 × 10
9
years.
Uranium Thorium Alpha
3-Actinium serious:-
89
Ac
227
-------------
88
Ra
226
+
+1
β
0
+
0
n
1
t
1/2
= 7.1× 10
8
years.
Actinium Radium Bata neutron
Some equations of radioactivity elements :-
1-
4
Be
9
+
2
He
4
----------------
6
C
12
+
0
n
1
.
2-
6
C
14
------------------------
7
N
14
+
-1
β
0
.
3-
7
N
14
+
2
He
4
--------------
9
F
18
.
4-
9
F
18
----------------------
8
O
17
+
1
H
1
.
5-
0
n
1
---------------------
P
+
+
-1
β
0
.
6- P
+
----------------------
0
n
1
+ е
+
.
7- е
+
+ P
+
----------------------
0
n
1
.
Effects of Ionizing radiation on living cells:-
1- Direct action of radiation -------
Ions(low energy)+ free radical (High
energy)----
(DNA) Mutant cells.
2- Indirect action (80%)H
2
O in cells-----
Ions +Free radicals ------ H
2
O ,
H
2
O
2
(toxic) ,H
2
+ free radicals.
Methods used to separation Isotopes:-
1- Gaseous diffusion :-depending on weight of isotopes
92
U
235
(High diffusion) ,
92
U
238
(Low diffusion).
2- Electromagnetic field .
3- Fractional distillation .
Sea water contain two types of isotopes (
1
D
2
,
1
H
1
) ,When
distillation this sample (sea water) give (
1
H
1
2
O) in first step but in
the last step give (
1
D
2
2
O).
4- Thermal diffusion method.
5- Chemical exchange :- Based on change of activation energy.
Methods of radiation therapy :-
1- External beam radiation therapy (used X-ray).
2- Internal beam radiation therapy (By used catalyst).
3- Mouth beam radiation therapy ( I
131
).
Medical uses of radioactive isotopes:-
1-To treat the different types of cancer tumors , for example
Thyroid gland by used Iodine -131 in (NaI)aq.
3- In photosynthesis method to limit the type of oxygen in glucose
(C
6
H
12
O
6
).
4- To limited the mechanical of reaction in organic chemistry.
5- In clinical field used to shown the arteries closed .
6- To limited the age of plants and animals .