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Modern Physics

Model of an atom

Model of atom

Protons Discovered by Goldstein  Neutrons > Protons > Electrons  (in terms of mass) 
Electrons Discovered by Thomson
Neutrons Discovered by Chadwick


  • When size of nucleus enlarges Electrostatic force > Nuclear force, which leads to radioactivity
  • Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability.


The decomposition process is called radioactivity. The energy and particles which are released during the decomposition process are called radiation.

  • When unstable nuclei decompose in nature, the process is referred to as natural radioactivity.
  • When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.

There are three major types of natural radioactivity

Alpha radiation

  • Consists of a stream of positively charged particles, called alpha particles
  • Alpha particles have an atomic mass of 4 and a charge of +2 (a helium nucleus).
  • When an alpha particle is ejected from a nucleus, the mass number of the nucleus decreases by four units and the atomic number decreases by two units. For example:


 23892U → 42He + 23490Th        (The helium nucleus is the alpha particle)

Beta Radiation

  • Consists of a stream of electrons, called beta particles.
  • When a beta particle is ejected, a neutron in the nucleus is converted to a proton, so the mass number of the nucleus is unchanged, but the atomic number increases by one unit. For example:


 23490Th    → 0-1e + 23491Pa (The electron is the beta particle)

 Gamma Radiation

  • Gamma rays are high-energy photons with a very short wavelength (0.0005 to 0.1 nm).
  • The emission of gamma radiation results from an energy change within the atomic nucleus.
  • Gamma emission changes neither the atomic number nor the atomic mass.
  • Alpha and beta emission are often accompanied by gamma emission, as an excited nucleus drops to a lower and more stable energy state.

Applications of Radioactivity

  • Used as a tracer for chemical reactions. You can put an isotope in a living organism and it will do the same reactions as the regular element but you will be able to trace what it reacts with and where it goes
  • Detecting how old something is by seeing how much of the isotope of the element is left Carbon Dating   C 14 (Used for living organisms) & Uranium dating   For non-living organism ex. rocks
  • Used for finding out the faults in metal structures esp. in airplanes radioactive material will penetrate more through the cracked areas
  • Act as a fuel for nuclear reactors to produce electricity
  • Some isotopes are used in the treatment of cancer to kill the cancer mutated cells
  • Some isotopes are used to study the proper functioning of internal organs
  • Gamma radiations are used to sterilize the surgical instruments
  • Radio phosphorous is used for studying the rate of phosphorous assimilation by the plant
  • Preservation of food grains and seeds
  • Used for preparing synthetic elements (artificial transmutation)
  • Detecting leaks in natural gas pipes

Nuclear fission

  • In nuclear fission the nucleus of an atom breaks up into two lighter nuclei.
  • The process is accompanied by the release of a large amount of energy.
  • The process may take place spontaneously in some cases or may be induced by the excitation of the nucleus with a variety of particles (e.g., neutrons, protons, deuterons, or alpha particles) or with electromagnetic radiation in the form of gamma rays.


Nuclear fission


Atomic bomb → Only by fissile U 235 i.e. Enriched Uranium (90%)

For Nuclear reactors 6 % of U-235


Nuclear fusion

  • Process by which nuclear reactions between light elements form heavier elements (up to iron).
  • During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy) substantial amounts of energy are released.


Nuclear fusion


Hydrogen bomb Requires an atomic bomb to detonate




Nuclear Reactor

  • A nuclear reactor, formerly known as atomic pile, is a device used to initiate and control a sustained nuclear chain reaction.
  • Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships.


Heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines. These either drive a ship’s propellers or turn electrical generators.

Nuclear Reactor

Sequential Process

Fission Controlled chain reaction of U 235 or Plutonium 239
Moderators D2O, H2O, Solid Graphite (To slow down neutrons bombardment & start a chain reaction)

Heat Generation Rotation of turbines Powering Generator Electricity through cable lines

Cooling Liquid sodium
Control Rods Cadmium (Which absorb excess neutrons)


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