## Tuesday, July 23, 2013

### Class XII, PHYSICS, Chapter # 19 "THE ATOMIC NUCLEUS"

NUCLEAR STRUCTURE
The nucleus consists of protons and neutrons. A protonis a positively charged particle having mass 1.6726 x 10(-27) kg and charge 1.6 x 10(-19) coulomb. The charge of the proton is equal in magnitude of the charge of an electron, but opposite to it in sign. Neutrons have no charge. Its mass is 1.6750 x 10(-31). The mass of proton is 1836 times the mass of an electron.
MASS NUMBER
The sum of the number of protons and neutrons in a nucleus is called Mass Number.
It is denoted by ‘A’. This number is also called Nucleus Number.
ATOMIC NUMBER
The number of protons in a nucleus is called Atomic Number or proton number or charge number.
It is denoted by ‘Z’.
NEUTRON NUMBER
The difference between mass number and atomic number is called Neutron Number.
It is denoted by ‘N’ and is given by
N = A – Z
REPRESENTATION OF AN ELEMENT
An element X having mass number A and atomic number Z is represented by the symbol zXA.
Where X is the chemical abbreviation for the particular element.
ISOTOPES
The elements having same atomic number but different mass number or neutrons number are called isotopes.
For example hydrogen deuterium and tritium
Hydrogen A = 1, Z = 1, N = 0
Deuterium A = 2, Z = 1, N = 1
Tritium A = 3, Z = 1, N = 2
Qs. Explain the phenomenon of radioactivity.
Introduction
Henri Bacqural discovered that Uranium atoms (z = 92) emit highly penetrating radiations that could penetrate paper, glass and even aluminium. On the basis of his experimental results, he explained the phenomenon of radiation.
Definition
The phenomenon of spontaneous disintegration of nucleus of atoms is known as radioactivity.
Explanation
Radioactivity is a self-disrupting activity exhibited by some naturally occurring elements. It has been found, that the elements with atomic number greater than 83 are unstable and emit certain type of radiations. Such substances (e.g. Uranium, Radium, Thorium) are called Radio-active substances and the radiations emitted from their nuclei are called radio active radiations and the phenomenon is known as Radioactivity. Rutherford and his co-workers proved that the radiations emitted by a radio active substance are of three different types.
Experiment
Radio Active radiations can be separated by applying electric or magnetic field to the element. A small amount of radioactive substance is placed at the bottom of a cavity drilled in a block of lead. When the narrow beam of radioactive rays is allowed to pass through the space between the two charged plates, the path of some rays bend. A similar effect is observed in the presence of magnetic field.
Results Obtained
The conclusion that were made fro the experiment are
1. α – Particles
The rays towards the negative plate indicate that they consist of positively charged particles. These were named as α-rays.
2. β – Particles
The rays bending towards the positive plate indicate that they consist of negatively charged particles. These were named as β (beta) rays.
3. γ – Rays
The rays that go undeflected indicate no charge and are therefore energetic photons or γ (gamma) rays.
Properties of α – Particles
1. α – Particles are Helium nuclei. The charge of a α-particle is twice the charge of a proton and its mass is four times than that of a-proton.
2. The speed of α-particles is 1/100 times the speed of light.
3. They produce fluorescence and effect the photographic plate.
4. α – Particles have low penetrating power.
5. They have high ionization power.
6. When a nucleus zXA disintegrates by the emission of an α-particles, its charge number (z) decreases by 2 and mass number (A) decreases by 4.
zXA —-> Z2 (VA.4) + α – Particle
Properties of β – Particles
1. β – Particles are electrons with more energy as compared to ordinary electrons because their origin is nucleus and not the atomic orbits.
2. The speed of β – particles is 1/10 times the speed of light.
3. They produce fluorescence and affect the photographic plate.
4. β – particles have greater penetrating power then α-particles.
5. They have low ionizing power.
6. When a nucleus zXA disintegrates by the emission of β – particle, its charge number (Z) decreases or increases by 1, while mass number remains same.
zXA —-> z+1 γA + -1βº (electron)
zXA —-> z-1 γA + +1βº (positron)
Properties of γ – Rays
1. γ – Rays are energetic photons and have no charge. They are similar to X – rays but more energetic.
2. They travel with the speed of light.
3. The produce fluorescence and affect the photographic plate.
4. Their penetrating power is very high.
5. They do not have any ionization power.
6. When γ – Rays emit out from the nucleus of a radio active substance, then the mass number (A) and charge number (Z) remain same
zXA —-> zXA + γ – Rays
Where zXA represents the nucleus in excited state.
Qs. Define and explain the law of radioactive decay. How can you determine the half life of a radioactive substance with the help of this law?
Statement
The rate of decay in a radioactive process is directly proportional to the number of parent nuclides, present in the unstable nuclides of the given species.
Mathematical Form
If ΔN be the number of nuclides disintegrated in time Δt and N be the number of Nuclides at time t, then:
ΔN ∞ N
ΔN ∞ Δt
=> ΔN ∞ NΔt
=> ΔN = -λNΔt
=> ΔN / Δt = -λN
Where is the decay constant and negative sign shows that number of atoms decrease w.r.t
=> 1/N ΔN = – λΔt
=> ΔN / N= – λΔt
HALF LIFE OF ELEMENT
Definition
It is the time in which half of radioactive elements decays from paront element to daughter element.
It is denoted by T 1/2.
Example
Suppose we have 10,000 radioactive atoms. If in 10 seconds, 50,000 of them decay, then this time is called the half life that radioactivity element.
Qs. Explain nuclear fission reaction also discuss its type.
Introduction
In 1943, Fermi, Serge and their co-workers studied the phenomenon of nuclear studies the phenomenon of nuclear reactions. On the basis of their experimental results they proposed a remarkable reaction. This was advanced by many scientist and fission reaction was discovered.
Definition
The process in which a heavy nucleus breaks up into two lighter nuclei of nearly equal masses after bombardment by a slow neutron is known as nuclear fission.
Explanation
When an isotope of uranium of 92U235 is bombarded with slow moving neutrons, then fission reactions takes place. During this process two new elements three neutrons and a large amount of energy is released. The two nuclei of new elements produced are Barium and Krypton. The nuclear fission reaction.
Barium and Krypton are known as Fission pigments, which are radio active. A large amount of heat energy is also liberated, which may be produced.
CHAIN REACTION
Fission reaction is a chain reaction that has been classified into the following two types.
1. Controlled Fission Chain Reaction.
2. Uncontrolled Fission Chain Reaction.
1. Controlled Fission Chain Reaction
In a fission reaction for one atom of uranium, three neutrons are produced, which may give rise to fission reaction in other uranium atoms. If two neutrons out of three are stopped then chain reaction takes place at uniform rate and a fixed amount of energy is obtained. This is done by usually Cadmium or graphite rods. In a nuclear reactor controlled chain reaction takes place.
2. Uncontrolled Fission Chain Reaction
If in a fission reaction, the number of neutrons is not controlled, then the reaction will build up at a very fast rate and in only few seconds, an explosion occurs. In an atom bomb, uncontrolled fission chain reaction takes place.
Qs. Define and explain the phenomena of Nuclear Fusion.
NUCLEAR FISSION
Definition
A process in which two light nuclei combine (or fuse together) to form a heavy nucleus and energy is released is called Nuclear Fusion.
The energy released is called Thermo-Nucleus Fusion Energy.
Explanation
For example when light nuclei of hydrogen are combined to form a heavier nucleus of helium energy is liberated. The final mass is smaller than the initial mass and the deficit of mass is comparatively greater than in fission. For this reason the energy liberated in the process of fission.
It is very difficult to produce fusion reaction due to the fact that when two positively charged nuclei are bought closer and closer and then fused together. Work has to be done against the electrostatic force of repulsion. This requires a great deal of energy.
Fusion reaction can produce great amount of energy. The raw material 1 the reaction is deuteron, which is found in abundance in world oceans as heavy water.
The fusion reaction is possible in sun and stars because of very high temperature. The fusion reactions are also the basic source of energy in stars including the sun.
This process is called Proton-Proton cycle. In this fusion process the amount of energy released is of the order of 25 MeV.
Another fusion process is suggested by Bethe. It is called Carbon-Nitrogen cycle or simply Carbon Cycle. This process is assumed to occurs in the sun. In this process four protons are converted into an alpha particle with carbon acting as a catalyst in the reaction