NUCLEAR CHEMISTRY
1.2.1 Radioactivity and Nuclear Reactions
1.2.2 Nuclear Stability
1.2.3 Energetics of Nuclear Reactions
1.2.4 Interaction of Radiation and Matter
Chapter 2
A chemical reaction occur as result of the interaction between valence electrons around an atom’s
nucleus. In 1896, Henri Becquerel, discovered radioactivity of chemical elements, thus calling it as
radiochemistry. He expanded the field of chemistry to include nuclear changes when he discovered
that uranium emitted radiation.
After Becquerel’s discovery, radioactivity was popularized by Marie Sklodowska Curie, together with
Pierre Curie, began studying radioactivity and completed much of the pioneering work on nuclear
changes. They found that there were two kinds of radioactive particles emitted from compounds.
They called the electrically charge negative (-) kind Beta (ß) particles and positive (+) kind alpha (𝛼)
particles.
RADIOACTIVITY
Radioactivity is the act of emitting radiation spontaneously. This is done by an atomic nucleus that,
for some reason, is unstable; it "wants" to give up some energy in order to shift to a more stable
configuration.
Radioactivity is a physical, not a biological, phenomenon. Simply stated, the radioactivity of a sample
can be measured by counting how many atoms are spontaneously decaying each second. This can be
done with instruments designed to detect the particular type of radiation emitted with each "decay"
or disintegration. The actual number of disintegrations per second may be quite large.
Scientists have agreed upon common units to use as a form of shorthand. Thus, a curie (abbreviated
"Ci" and named after Pierre and Marie Curie, the discoverers of radium[87]) is simply a shorthand
way of writing "37,000,000,000 disintegrations per second," the rate of disintegration occurring in 1
gram of radium. The more modern International System of Measurements (SI) unit for the same type
of measurement is the Becquerel (abbreviated "Bq" and named after Henri Becquerel, the discoverer
of radioactivity), which is simply a shorthand for "1 disintegration per second."
ISOTOPES
Isotopes [ahy-suh-tohps] are atoms with the same number of protons, but differing numbers
of neutrons. In other words, they have different atomic weights. Isotopes are different forms of a
single element. There are 275 isotopes of the 81 stable elements. There are over 800 radioactive
isotopes, some of which are natural and some synthetic.
Every element on the periodic table has multiple isotope forms.
The chemical properties of isotopes of a single element tend to be nearly identical. The exception
would be the isotopes of hydrogen since the number of neutrons has such a significant effect on the
size of the hydrogen nucleus. The physical properties of isotopes are different from each other since
these properties often depend on mass. This difference may be used to separate isotopes of an
element from each other by using fractional distillation and diffusion.
NUCLEAR REACTION
A nuclear reaction is considered to be the process in which two nuclear particles (two nuclei or a
nucleus and a nucleon) interact to produce two or more nuclear particles or ˠ-rays (gamma rays).
Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. Sometimes
if a nucleus interacts with another nucleus or particle without changing the nature of any nuclide,
the process is referred to as nuclear scattering, rather than a nuclear reaction. Perhaps the most
notable nuclear reactions are the nuclear fusion reactions of light elements that power the energy
production of stars and the Sun. Natural nuclear reactions occur also in the interaction between
cosmic rays and matter.
Most chemical reactions involve an element’s outer electrons as they are shared, swapped and
bumped. Nuclear reactions are different. All the action takes place inside the nucleus.
There are two types of nuclear reactions. The first is the radioactive decay of bonds within the
nucleus that emit radiation when broken. The second is the “billiard ball type of reaction where a
nucleus or a nuclear particle (like proton) collides with another nucleus or nuclear particle.
1. RADIOACTIVE DECAY
Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting energy in the
form of emitted particles or electromagnetic waves, called radiation.
Some isotopes of a given element are more unstable than others, causing a nuclear reaction
which releases energy to achieve a more stable nuclear configuration. Such isotopes are
radioactive, and are referred to as “radioisotopes.”
This process gives off energy in the form of ionizing particles or radiation. No collision with
other atoms is needed, it just happens spontaneously. The original atom is called the parent
nuclide, while the atom after emission is called the daughter nuclide.
Example, carbon 14 (parent nuclide) emits radiation to become nitrogen 14 (daughter nuclide).
While all radioactive isotopes go through this process, each one has its own decay rate.
TYPES OF DECAY
The three types of radiation have different levels of penetrating power. Penetrating power refers
to the energy with which the radiation particles are ejected from the atom. The higher the
energy, the more the particles or light produced by radioactive decay will penetrate a substance.
⮚ ALPHA DECAY
An alpha particle (α\alpha) is made up of two protons and two neutrons bound together. This
type of radiation has a positive charge (due to the presence of two protons). An alpha particle
is sometimes represented using the chemical symbol He 2+, because it has the same structure
as a helium atom missing its two electrons—hence the overall charge of +2. Their massive size
(compared to beta particles, for instance) means alpha particles have very low penetration
power. Penetration power describes how easily the particles can pass through another
material. Since alpha particles have a low penetration power, the outside layer of the human
skin, for example, can block these particles.
Alpha particle/decay are commonly given off or emitted by larger radioactive isotopes,
examples are uranium, thorium, actinium, radium and transuranium elements.
Alpha decay occurs because the nucleus of a radioisotope has too many protons. A nucleus
with too many protons causes repulsion between these like charges. To reduce this repulsion,
the nucleus emits an α particle. Examples of this can be seen in the decay of americium (Am)
to neptunium (Np).
⮚ BETA DECAY
In radioactive nuclei with too many neutrons, a neutron can be converted into an electron,
called beta particle. Beta particles (β) have a higher penetration power than alpha particles
(they are able to pass through thicker materials such as paper).
During beta decay, the number of neutrons in the atom decreases by one, and the number of
protons increases by one. Effectively, a neutron was converted into a proton in the decaying
nucleus, in the process releasing a beta particle. Since the number of protons before and after
the decay is different, the atom has changed into a different element.
As mentioned above, beta particles emitted as radiation are high energy, high speed electrons
or positrons that came from the nucleus. When atoms like potassium 40 give off beta particles,
there is a net charge of zero. This process happens in three different ways:
● Neutron-proton ratio increase, so a neutron (neutral) changes into a proton and release
an electron.
● Neutron-proton ratio decrease, so a proton changes into a neutron and releases a
positron (beta particle)
● Neutron-proton ratio in the nucleus decrease, so the nucleus captures an electron and
changes a proton into a neutron.
Beta particles have average ionizing and penetrating power, they are between the strength of
alpha particles and gamma rays. Beta particles, particularly strontium-90, are used for
treating eye and bone cancers, Positron or beta plus decay, can be useful as a tracer during
positron emission tomography, as in a PET scan.
⮚ GAMMA DECAY
Some decay reactions release energy in the form of electromagnetic waves called gamma rays.
Gamma radiation (γ) is part of the electromagnetic spectrum, just like visible light. However,
unlike visible light, humans cannot see gamma rays, because they have a much higher
frequency and energy than visible light. Gamma radiation has no mass or charge. This type of
radiation is able to penetrate most common substances, including metals. The only substances
that can absorb this radiation are thick lead and concrete.
Gamma decay reactions occur if the energy of the radioisotope’s nucleus is too high, and the
resulting atomic number and atomic mass remain unchanged during the course of the reaction.
When a radioactive element decays, different nuclear particles are given off. These
radioactive particles can be separated by an electric (magnetic field) and detected in the
laboratory.
Alpha (
) particles = positively (+) charged particles
Beta (ß) particles = negatively (-) charged particles
Gamma ( ) particles = high energy particles with zero charge
NUCLEAR STABILITY
The nucleus is made of neutrons and protons. What causes them to stick together? Why do the protons
not repel each other?
Nuclear Stability refers to the elements whose nucleus is stable, it means that the nuclear stability
denotes the stability of the elements. Those atoms which have stable nucleus are known as nuclear
stable atom. It also helps to identify the stability of an isotope.
The two main factors that determine nuclear stability are the neutron/proton ratio and the total
number of nucleons in the nucleus.
ISOTOPE
Isotope is an element that has the same atomic number but different atomic mass compared to the
periodic table. Every element has a proton, neutron, and electron. The number of protons is equal
to the atomic number, and the number of electrons is equal to the protons, unless it is an ion.
To determine the number of neutrons in an element you subtract the atomic number from the atomic
mass of the element. Atomic mass is represented as (AA) and atomic number is represented as (ZZ)
and neutrons are represented as (NN).
A=N+Z
A=N+Z
The principal factor for determining whether a nucleus is stable is the neutron to proton ratio.
Elements with (Z<20Z<20) are lighter and these elements' nuclei have a ratio of 1:1 and prefer to
have the same amount of protons and neutrons.
EXAMPLE 1.11.1: CARBON ISOTOPES
Carbon has three isotopes that scientists commonly used: C12C12, C13C13, C14C14. What is the
number of neutron, protons, total nucleons and N:ZN:Z ratio for the C12C12 nuclide?
SOLUTION
For this specific isotope, there are 12 total nucleons (AA. From the periodic table, we can see
that ZZ for carbon (any of the isotopes) is 6, therefore N=A-Z (from Equation 1):
12−6=6(1.2)(1.2)12−6=6
The N:P ratio therefore is 6:6 or a 1:1. In fact 99% of all carbon in the earth is this isotope.
CONNECTION BETWEEN NUCLEAR STABILITY AND RADIOACTIVE DECAY
The nuclei of radioisotopes are unstable. In an attempt to reach a
more stable arrangement of its neutrons and protons, the unstable
nucleus will spontaneously decay to form a different nucleus. If
the number of neutrons changes in the process (number of protons
remains), a different isotopes is formed and an element remains
(e.g. neutron emission). If the number of protons changes
(different atomic number) in the process, then an atom of a
different element is formed.
This decomposition of the nucleus is referred to as radioactive
decay. During radioactive decay an unstable nucleus spontaneously and randomly decomposes to
form a different nucleus (or a different energy state – gamma decay), giving off radiation in the form
of atomic particles or high energy rays. This decay occurs at a constant, predictable rate that is
referred to as half-life. A stable nucleus will not undergo this kind of decay and is thus nonradioactive.
ENERGETICS OF NUCLEAR REACTIONS
INTERACTION OF RADIATION AND MATTER
During the radioactive decay process, a particle and/or a photon are emitted from the parent atom.
The particles emitted carry energy proportional with their mass and speed. The photons carry energy
proportional to their frequency. The photons and the particles interact with the surrounding matter.
This interaction depends on their type (alpha, beta, gamma, neutrons, etc.), mass, electrical charge,
energy, and on the composition of the surrounding materials.
X-ray photons are created by the interaction of energetic electrons with matter at the atomic level.
Photons (x-ray and gamma) end their lives by transferring their energy to electrons contained in
matter. X-ray interactions are important in diagnostic examinations for many reasons. For example,
the selective interaction of x-ray photons with the structure of the human body produces the image;
the interaction of photons with the receptor converts an x-ray or gamma image into one that can be
viewed or recorded. This chapter considers the basic interactions between x-ray and gamma photons
and matter.
TYPES OF INTERACTIONS
⮚ PHOTON INTERACTION
Photons are individual units of energy. As an x-ray beam or
gamma radiation passes through an object, three possible fates
await each photon, as shown in the figure at the right side:
1. It can penetrate the section of matter without interacting.
2. It can interact with the matter and be completely absorbed by
depositing its energy.
3. It can interact and be scattered or deflected from its original
direction and deposit part of its energy.
Photons Entering the Human Body Will Either Penetrate, Be
Absorbed, or Produce Scattered Radiation
There are two kinds of interactions through which photons deposit their energy; both are with
electrons. In one type of interaction the photon loses all its energy; in the other, it loses a portion
of its energy, and the remaining energy is scattered. These two interactions are shown below.
Photoelectric
▪ A photon transfers
all its energy to an
electron which is
located on one of the
atomic shells.
▪ The following are
the effects:
o Electrons are
ejected from the
atom and starts to
pass to a nearby
matter.
o Causing it to rapidly
lose its energy and transfer in a short distance from its original location.
o The energy of the photon then will be deposited in the matter which is close to
the location of the photoelectric interaction.
▪
Basis for the reaction to occur:
o When electrons are firmly bound to the atom
o With a high binding energy which is a probable reason, they should be only slightly
less than the energy of the photon, otherwise it will not occur.
o Take note that this interaction is possible only when the photon has sufficient energy
to overcome the binding energy and remove the electron from the atom.
Compton Interaction
▪ In this interaction a portion of energy is absorbed only and the photon is formed with
reduced energy.
▪ This photon leaves the site of the interaction in a direction different from that of the
original photon.
▪ Because of the change in photon direction, this type of interaction is classified as a
scattering process. In effect, a portion of the incident radiation "bounces off' or is scattered
by the material.
Coherent Scatter
▪ This is pure scattering interaction and there is no energy deposits in the material. This
is possible only when the photon produces low energy and it cannot be used in most
diagnostic procedure.
Pair Production
▪ It only occurs in photons with energies in excess of 1.02 MeV.
▪ In this interaction the photon interacts with the nucleus wherein its energy is converted
to matter. It produces a pair of particles which are the electron and a positively
charged positron, they have the same mass, and each has an equivalent to a mass
energy of 0.51 MeV.
▪ Like coherent scatter this is not used in diagnostic procedure as well.
POSITRON INTERACTION
Positron has a positive charge and is the same in size with that of the electron and it is different
from an electron because they are composed of antimatter. This leads to a type of interaction that
is quite different from the interactions among electrons.
The interaction between a positron and matter is in two phases:
▪ ionization
o as the energetic positron passes through matter, it interacts with the atomic electrons
by electrical attraction. As the positron moves along, it pulls electrons out of the atoms
and produces ionization. A small amount of energy is lost by the positron in each
interaction. In general, this phase of the interaction is not too unlike the interaction of
an energetic electron, but the positron pulls electrons as it races by and electrons push
electrons away-from the path. Also, when the positron has lost most of its kinetic
energy and is coming to a stop, it comes into close contact with an electron and enters
into an annihilation interaction.
▪
Annihilation.
o The annihilation process occurs when the antimatter positron combines with the
conventional-matter electron. In this interaction, the masses of both particles are
completely converted into energy.
o The relationship between the amount of energy and mass is given by
E = mc2
o The energy equivalent of one electron or positron mass is 511 keV. The energy that
results from the annihilation process is emitted from the interaction site in the form of
two photons, each with an energy of 511 keV.
o The pair of photons leave the site in opposite directions. With special imaging
equipment it is possible to capture both photons and to determine the precise threedimensional location of the interaction site. Since the range of a positron, like that of
an electron, is relatively short, the site of interaction is always very close to the
location of the radioactive nuclei.
FUELS
Fuel is a substance which gives heat energy on combustion. A fuel contains carbon and hydrogen as
main combustible elements. Fuel is any material that can be made to react with other substances so
that it releases chemical or nuclear energy as heat or to be used for work.
Heat energy released by reactions of fuels is converted into mechanical energy via a heat engine.
Other times the heat itself is valued for warmth, cooking, or industrial processes, as well as the
illumination that comes with combustion. Fuels are also used in the cells of organisms in a process
known as cellular respiration, where organic molecules are oxidized to release usable energy.
CLASSIFICATION OF FUEL
o BASED ON PHYSICAL STATE
o BASED ON OCCURENCE
BASED ON PHYSICAL STATE
Liquid Fuels
▪ Liquid fuels like furnace oil are predominantly used in industrial
applications. Most liquid fuels in widespread use are derived from the
fossilized remains of dead plants and animals by exposure to heat and
pressure in the Earth's crust. However, there are several types, such as
hydrogen fuel (for automotive uses), ethanol, jet fuel and biodiesel
which are all categorized as a liquid fuel.
▪ Types of liquid fuel
o Petroleum
o Oils from distillation of petroleum
o Coal tar
o Shale-oil
o Alcohols, etc.
▪
liquid fuels has the following advantages and Disadvantages:
Advantages:
possess higher calorific value per unit mass as compare to solid fuels.
out any loss.
cost of liquid fuel is relatively much higher as compared to solid fuel.
Disadvantages:
bad odour.
ners and spraying apparatus are required for efficient burning of liquid
fuels.
Solid Fuels
▪ Solid fuel refers to various types of solid material that are
used as fuel to produce energy and provide heating,
usually released through combustion.
▪ Coal is classified into three major types; anthracite,
bituminous, and lignite. However, there is no clear
demarcation between them. Coal is further classified as
semi-anthracite, semi-bituminous, and sub-bituminous.
Anthracite is the oldest coal from a geological
perspective. It is a hard coal composed mainly of carbon
with little volatile content and practically no moisture.
▪ Types of solid fuel
o Wood, Coal Oilshal Tanbark, Bagasse,Straw,
Charcoal, Coke, Briquettes
▪ Solid fuels has the following advantages and Disadvantages:
Advantages
explosion.
burn with clinker formation.
Disadvantages
Gaseous Fuel
▪ Fuel gas is any one of a number of fuels that under ordinary conditions are gaseous. Many fuel
gases are composed of hydrocarbons, hydrogen, carbon monoxide, or mixtures thereof. Such
gases are sources of potential heat energy or light energy that can be readily transmitted and
distributed through pipes from the point of origin directly to the place of consumption. Fuel
gas is contrasted with liquid fuels and from solid fuels, though some fuel gases are liquefied
for storage or transport. While their gaseous nature can be advantageous, avoiding the
difficulty of transporting solid fuel and the dangers of spillage inherent in liquid fuels, it can
also be dangerous.
▪ Types of gaseous fuel
o Natural gas
o Liquefied Petroleum gas (LPG)
o Refinery gases
o Methane from coal mines
o Fuel gases made from solid fuel
o Gases derived from coal
o Gases derived from waste and biomass
o Blast furnace gas
o Gases made from petroleum
o Gases from oil gasification
o Gases from some fermentation process
▪ Gaseous fuels has the following advantages and Disadvantages over solid or liquid fuels :
Advantages
transportation.
have high heat contents therefore provides higher temperatures.
-heated by the heat of hot waste gases.
Disadvantages
PROPERTIES OF GASEOUS FUELS
The fuel should be compared based on their net calorific value and especially true for natural gas
because increased hydrogen content results in high water formation during combustion.
1. LPG
▪ LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure
but may be condensed to the liquid state at normal temperature by the application of
moderate pressures. The LPG is a predominant mixture of propane and butane with a small
percentage of unsaturated, some lighter C2 and heavier C5 fractions. The propane (C3H8),
Propylene (C3H6), iso-butane (C4H10) and Butylene (C4H8) are included in the range of LPG.
The liquid LPG evaporates to produce about 250 times volume of gas.
▪
LPG vapor is denser than air for example butane is about two times heavier then air and
propane is about 1.5 times heavier then air. Consequently the vapors may flow along the
ground and into drains sinking to the lowest level of the surroundings and be ignited at a
considerable distance from the source of leakage. There should be adequate ground level of
ventilation where LPG is stored therefore LPG cylinders should not be stored in cellars or
basements which have no ventilation at ground level.
2. Natural gas
▪ Natural gas has high calorific value and requiring no storage facilities. It mixes with air readily
and does not produce smoke or soot. It did not contains Sulphur. It is lighter than air and
disperses into air easily in case of leak.
▪ Methane is the main constituent of natural gas and it is about 95% of the total volume. The
other components are Ethane, Propane, Butane, Pentane, Nitrogen, Carbon Dioxide, and
traces of other gases. In these gases a very small amounts of sulphur compounds are also
present. The properties of methane are used when comparing the properties of natural gas to
other fuels because methane is the largest component in natural gas.
BASED ON OCCURENCE
▪
PRIMARY OR NATURAL FUEL
wood
coal
▪
SECONDARY OR PREPARED FUELS
charcoal
petroleum