Atomic & Nuclear Physics

Atomic Physics

  • E. Rutherford is known as the father of nuclear physics.
  • Atom is the smallest part of matter which takes part in chemical reactions. Atoms of the same element are similar in mass, size and characteristics. Atom consists of three fundamental particles electron, proton and neutron. All the protons and neutrons are present in the central core of atom called nucleus. Electrons revolve around the nucleus.
  • In an atom, electrons and protons are equal in number and have equal and opposite charge. Hence atom is neutral.

Properties of Fundamental Particles

Particle

Mass (kg)

Charge (Coulomb)

Discoverer

Proton

1.672 X 10-27

+1.6 \times 10^{-19}

Rutherford

Neutron

1.675 X 10-27

0

Chadwick

Electron

9.108 X 10-31

-1.6 \times 10^{-19}

J.J. Thomson

Note: Proton was discovered by Goldstein and named by Rutherford.

  • Till today, several subatomic particles have been discovered. Some important of them are as follows.

Particle

Mass (kg)

Charge (Coulomb)

Discoverer

Discovery

Positron

9.108 X 10-31

+1.6 \times 10^{-19}

Anderson

Antiparticle of electron

Neutrino

0

0

Pauli

 

Pi-meson

274 times the mass of electron

Positive and negative both

Yakawa

Unstable

Photon

0

0

 

Velocity equal to that of light

Cathode Rays:

If the gas pressure in a discharge tube is 10^{-2} to 10^{-3} mm of Hg and a potential difference of 10^4 volt is applied between the electrodes, then a beam of electrons emerges from the cathode which is called cathode rays. Hence cathode rays are beam of high energy electrons. Cathode is an electrode with a negative charge.

Properties of cathode rays:

  • Cathode rays are invisible and travel in straight line.
  • These rays carry negative charge and travel from cathode to anode.
  • These rays emerge perpendicular to the cathode surface and are not affected by the position of anode.
  • Cathode rays travel with very high velocity (1/ 10ththe velocity of light).
  • These rays are deflected by electric and magnetic fields.
  • These rays can ionise gases.
  • These rays heat the material on which they fall.
  • They can produce chemical change and thus affect a photographic plate.
  • These rays can penetrate through thin metal foils.
  • The source of emf used in the production of cathode rays is induction coil.
  • When they strike a target of heavy metals such as tungsten, they produce x-rays.
  • The nature of cathode rays is independent of nature of cathode and the gas in the discharge tube.

Positive or Canal rays:

If perforated cathode is used in a discharge tube, it is observed that a new type of rays are produced from anode moving towards the cathode and passed through the holes of cathode. These rays are positively charged and are called positive rays or canal rays or anode rays. These rays were discovered by Goldstein.

Properties of Canal rays:

  • The positive rays consists of positively charged particles.
  • These rays travel in straight line.
  • These rays can exert pressure and thus possess kinetic energy.
  • These rays are deflected by electric and magnetic fields.
  • These rays are capable of producing physical and chemical changes.
  • These rays can produce ionisation in gases.

Radioactivity

  • Radioactivity is the sending out of harmful radiation or particles, caused when atomic nuclei breakup spontaneously.
  • Radioactivity was discovered by Henry Becquerel for which he received the Nobel Prize of Physics jointly with Madame Curie and Pierre Curie.
  • The nucleus having protons 83 or more are unstable. They emit α,β  and γ  particles and become stable. The elements of such nucleus are called radioactive elements and the phenomenon of emission of α,β  and γ  particles is called radioactivity.
  • β rays are fast moving electrons. In the nucleus an electron is created due to conversion of neutron into proton.
  • γ  rays are electromagnetic waves. 7rays are emitted after the emission of α  and β rays.
  • Robert Pierre and his wife Madame Curie discovered a new radioactive element radium.
  • The rays emitted by

S.

Stable nucleus

Unstable nucleus

1.

Low atomic number

Hight atomic number.

2.

Low mass number

High mass number

3.

Nuclus of small size

Nucleus of bigger size

4.

\frac{n}{p}= 1

\frac{n}{p} > 1

Properties of α,β and γ particles

Properties

α

β

γ

Origin

Nucleus

Nucleus

Nucleus

Nature

Positively charged

Negatively charged

Neutral

Composition

{He^{4}

 _1e^0

Photon

Mass ‘'

6.4 \times 10^{-27} Kg

9.1 \times 10^{-31} Kg

zero

Charge

 +2e

-e

zero

Chemical effect

Affects photo graphic plate

Affects photo graphic plate

Affects photo graphic plate

Effect of electric and magnetic field

Deflected

Deflected

No effect

Penetrating  power

Minimum

In between the other two

Maximum

Ionising Power

Minimum

In between the other two

Maximum

Velocity

Between 1.4 \times 10^{7}

m/s to 2.2 \times 10^{7}

m/s

1% to 99% of velocity of light

3 \times 10^{8}m/s

 

  • With the emission α -particle, atomic number is decreased by 2 and mass number is decreased by 4.
  • With the emission of a β -particle atomic number is increased by one and mass number does not change.
  • The effect on the mass number and atomic number with the emission of α,β  and γ  rays is decided by Group-displacement law or Soddy-Fajan Law.
  • Radioactivity is detected by G.M. Counter.
  • The time in which half nuclei of the element is decayed is called half life of the radioactive substance.
  • Cloud chamber: Cloud chamber is used to detect the presence and kinetic energy of radioactive particles. It was discovered by C.R.T. Wilson.
  • Radioactive carbon-14 is used to measure the age of fossils and plants. (Carbon dating) In this method age is decided by measuring the ratio of _6{C^{12}} and _6{C^{14}}

Nuclear Fission and Fusion

  • A nucleus is represented as  _z{X^{A}} where Z is the number of proton (called atomic number), A is sum of number of protons (Z) and number of neutrons (N) and is called mass number.
  • Number of neutrons N = A - Z For example,  __{92}{U^{238}} has 92 protons, 238 - 92 = 146 neutrons and 238 nucleons (protons + neutrons).

Isotopes—Atoms having same atomic number but different mass number are isotopes.

Isobars—Atoms having same mass number but different atomic number are isobars.

Isotones—Elements having same number of neutron but different atomic number are isotones.

Nuclear Fission:

The nuclear reaction in which a heavy nucleus splits into two nuclei of nearly equal mass is nuclear fission. The energy released in the nuclear fission is called nuclear energy.

  • Nuclear fission was first demonstrated by Strassmann and O. Hahn. They found that when  {U^{235}} nucleus is excited by the capture of a neutron, it splits into two nuclei  {Ba^{142}} {K^{92}}.

Chain Reaction:

When uranium atom is bombarded with slow neutrons, fission takes place. With the fission of each uranium nucleus, on the average 3 neutrons and large energy is released. These neutrons cause further fission. Clearly a chain of fission of uranium nucleus starts which continues till whole of uranium is exhausted. This is called chain reaction.

Chain reaction is of the following two types-

1. Uncontrolled chain reaction 2. Controlled chain reaction

1. Uncontrolled Chain Reaction:

In each fission reaction, three more neutrons are produced. These three neutrons may cause the fission of three other  {U^{235}} nuclei producing 9 neutrons and so on. As a result the number of neutrongoes on increasing till the whole of fissionable material is consumed. This chain reaction is called uncontrolled or explosive chain reaction. This reaction proceeds very quickly and a huge amount of energy is liberated in a short time.

Atom bomb: Atom bomb is based on nuclear fission. {U^{235}} and Pu 239 {Pu^{239}} are used as fissionable material. This bomb was first used by USA against Japan in second world war (6th August, 1945 at Hiroshima & 9th August, 1945 at Nagashaki).

2. Controlled Chain Reaction:

A fission chain reaction that proceeds slowly without any explosion and in which the energy released can be controlled is known as controlled reaction. Actually in this situation only one of the neutrons produced in each fission is able to cause further fission. In controlled chain reaction, the rate of reaction remains constant.

Nuclear Reactor or Atomic Pile:

Nuclear reactor is an arrangement in which controlled nuclear fission reaction takes place.

First nuclear reactor was established in Chicago University under the supervision of Prof. Fermi.

There are several components of nuclear reactor which are as follows:
1. Fissionable Fuel:  {U^{235}}  or  {U^{239}}  is used.

2. Moderator: decreases the energy of neutrons so that they can be further used for fission reaction. Heavy water and graphite are used as moderator.

3. Control rod: Rods of cadmium or boron are used to absorb the excess neutrons produced in fission of uranium nucleus so that the chain reaction continues to be controlled.

4. Coolant: A large amount of heat is produced during fission. Coolant absorbs that heat and prevents excessive rise in the temperature. The coolant may be water, heavy water (D_2Odeuterium (_1H^2) is an isotope of hydrogen), or a gas like He or CO_2.

Uses of nuclear reactor

1. To produce electrical energy from the energy released during fission.

2. To produce different isotopes which can be used in medical, physical and agriculture science.

Fast Breeder Reactor: A nuclear reactor which can produce more fissionable fuel than it consumes is called a fast breeder reactor.

Nuclear Fusion:

When two or more light nuclei combined together to form a heavier nucleus, tremendous energy is released. This phenomenon is called nuclear fusion. A typical example of nuclear fission is
 _1H^2 + _1H^3 \rightarrow _2He^4 + _on^1 + 17.6 Mev

  • The energy released by sun and other stars is by nuclear fusion.
  • For the nuclear fusion, a temperature of the order of 10^8 K is required.

Hydrogen bomb:

Hydrogen bomb was made by American scientists in 1952. This is based on nuclear fusion. It is 1000 times more powerful than atom bomb.

Mass Energy Relation:

In 1905 Einstein established a relation between mass and energy on the basis of special theory of relativity. According to this relation, mass can be converted into energy and vice versa, according to the relation \textbf{E} = \textbf{mc}^\textbf{2} where c is the velocity of light and E is the energy equivalent of mass m.

  • Albert Einstein was an American scientist. He was born in Germany. He was given Nobel Prize of Physics in 1921.
  • Sun is continuously emitting energy. Earth is continuously receiving 4 \times 10^^{26} joule of energy per second from sun. As a result mass of sun is decreasing at the rate of approximately 4 \times 10^^{9} kg per second. But mass of sun is so large that it is estimated that the sun will continuously supply energy for next 10^{9} years.

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