Binding Energy
The nucleus of
an atom is made of protons ( +
charge) and neutrons ( No charge). Then
why don’t the protons repel each other? What causes these to stick together?
In fact, the
mass of the nucleus is less than the sum of the masses of the individual
protons(p) and neutrons (n) which makeup
the nucleus.
The lost mass
(mass defect) get changed into the energy which is necessary to bind the
nucleus together. Suppose the mass defect is ∆m. Then the energy binding the
nucleus together ∆E is given by:
∆E = ∆mc2 where c is the speed of light.
If is divided it by the number of nucleons in
the nucleus, then the binding energy per nucleon is obtained. The higher this
value is the more stable is the nucleus.
Stable Nuclides
Since the positive protons in the nucleus
repel each other, neutrons are required to bind the nucleus together. For
lighter elements, the number of protons and neutrons are approximately equal.
As the number of protons increases the force required to hold the nucleus
together increases and to such an extent that more neutrons are required and
the ratio of neutrons / protons is greater than one. For heavier elements, the
number of neutrons in the nucleus is approximately one and a half times the
number of protons.
Unstable Nuclides
Whenever a β -particle
is emitted a neutron is lost and a proton gained. Therefore the neutron/proton
ratio is reduced by β- radiation.
When an α -particle is emitted the nucleus loses two
protons and two neutrons. Since there were originally more neutrons than
protons it follows that the neutron/proton ratio is increased.
Nuclear Fission
Radioactive
elements can undergo a number of different reactions in the process of forming
a stable element. Nuclear fission is one such reaction. Fission = To divide.
Elements having atomic numbers greater than 90 can undergo fission. Uranium is
one such element. In nuclear fission, the nucleus of an atom splits up.
In other words,
heavy unstable nuclides can be split to produce energy in a process called nuclear
fission. When uranium decays naturally α
- particles and β - particles are
emitted. However, when uranium - 235 is bombarded by slow moving neutron it
forms uranium – 236and turns its nucleus unstable. Uranium - 236 breaks down,
splitting into two large particles and emitting three neutrons. When the exact
masses of the final products are added , the sum is found to be appreciably
less than the sum of the exact masses of the uranium - 235 and the original
neutron. This difference in mass ∆m appears as energy given by
∆E = ∆mc2 where c is the speed of light.
Fission in
uranium atoms may result in a chain reaction. A nuclear chain reaction is a
series of rapid nuclear fissions. A small sample of uranium contains billions
of atoms. When one U- 235 nucleus is split by a neutron, it releases three
neutrons. These neutrons are used to split three more uranium nuclei. Each
nuclei, causing them to split. The fission of nuclei and release of neutrons
becomes a chain reaction. In a chain reaction, billions of fission reaction may
occur per second.
NUCLEAR FUSION
When lighter
nuclides fuse together to form a heavy nucleus the process is called nuclear
fusion. In this energy is produced and mass is lost. For example, two atoms of
heavy hydrogen may fuse together to form helium and a neutron:
The sum of the
exact masses of the helium atom and the neutron is less than the sum of exact
masses of the two heavy hydrogen atoms. This lost mass is released as energy.
It is thought that the Sun's energy is produced by nuclear fusion.
Nuclear fusion
is the opposite of nuclear fission. Elements with small masses combine to form
elements with larger masses.
For nuclear
fusion to occur, temperatures even more than one million degree Celsius must be
reached. Nuclear fusion is called thermonuclear reaction. At the tremendous
temperature of thermonuclear reactions, nuclear atoms no longer exist. The
atoms lose their electrons (ionize) and become plasma. Then nuclei have enough
energy to overcome forces of repulsion between them.
The temperature
conditions for fusion exists in the sun and stars. The sun has an internal
temperature of about 20,000,000oC. In the sun, fusion occurs through
a complex series of nuclear changes. During the early life of a star, four
hydrogen nuclei are fused into one helium nucleus in a series of steps.
The sun is
constantly losing hydrogen as it forms helium. A tremendous amount of energy is
released during this fusion. The energy comes from matter that is converted to
energy. The helium that is formed has a mass almost one percent less than the
mass of the four hydrogen atoms. This one percent mass is converted to energy.
