Structure of atom

• The rich diversity of chemical behaviour of different elements can be traced to the differences in the internal structure of atoms of these elements
• The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who were of the view that atoms are the fundamental building blocks of matter
• The word 'atom' has been derived from the Greek word 'a-tomio' which means 'uncut-able' or 'non-divisible'
• These earlier ideas were mere speculations and there was no way to test them experimentally
• The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school teacher in 1808
• Dalton's atomic theory

  • Regarded the atom as the ultimate particle of matter
  • Able to explain the law of conservation of mass, law of constant composition and law of multiple proportion very successfully
  • Failed to explain the results of many experiments
• Atoms are made of sub-atomic particles, i.e., electrons, protons and neutrons — a concept very different from that of Dalton
• Like charges repel each other and unlike charges attract each other
• Cathode rays

  Particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode)
• Cathode rays are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them
• In the absence of electrical or magnetic field, cathode rays travel in straight lines
• In the presence of electrical or magnetic field, the behaviour of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons
• The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube
• Electrons are basic constituent of all the atoms
• Charge to mass ratio of electron (e/me)

  1.758820 × 1011 C kg–1
• Charge on the electron (e)

  • 1.602176 × 10–19 C
• Mass of the electron (me)

  9.1094 × 10–31 kg
• Positively charged particles discovered in electrical discharge tubes are called canal rays
• Canal rays


  • Mass depends on the nature of gas present in the cathode ray tube
  • Charge to mass ratio depends on the gas from which these originate
  • Some carry a multiple of the fundamental unit of electrical charge
  • Behaviour in magnetic or electrical field is opposite to that of electrons
• The smallest and lightest positive ion obtained from hydrogen was called proton
• Electrically neutral particles slightly heavier than protons were discovered by Chadwick and named as neutrons
• Fundamental particles and their properties

  • Electron: Charge -1.602176 × 10–19 C, Mass 9.109382 × 10–31 kg
  • Proton: Charge +1.602176 × 10–19 C, Mass 1.6726216 × 10–27 kg
  • Neutron: Charge 0, Mass 1.674927 × 10–27 kg
• After the discovery of sub-atomic particles, scientists faced problems in accounting for the stability of atom, comparing the behaviour of elements, explaining the formation of different molecules, and understanding the origin and nature of electromagnetic radiation absorbed or emitted by atoms
• Thomson model of atom

  Atom possesses a spherical shape with positive charge uniformly distributed and electrons embedded in it
• Thomson's model was able to explain the overall neutrality of the atom, but was not consistent with the results of later experiments
• 1. rays were discovered by Wilhalm Röentgen in 1895
• Radioactivity was observed by Henri Becqueral, and three types of rays (alpha, beta and gamma) were discovered
• Alpha particles were found to be helium nuclei, beta rays are negatively charged particles, and gamma rays are electromagnetic radiation
• α-particles


  Particles carrying two units of positive charge and four units of atomic mass
• α-particles are helium nuclei as when α-particles combined with two electrons yielded helium gas
• β-rays


  Negatively charged particles
• Thomson model of atom


  • Can be visualised as a pudding or watermelon of positive charge with plums or seeds (electrons) embedded into it
  • The mass of the atom is assumed to be uniformly distributed over the atom
• The Thomson model was able to explain the overall neutrality of the atom, but was not consistent with the results of later experiments
• Thomson was awarded Nobel Prize for physics in 1906, for his theoretical and experimental investigations on the conduction of electricity by gases
• Properties of fundamental Particles

  • Name
  • Symbol
  • Absolute charge/C
  • Relative charge
  • Mass/kg
  • Mass/u
  • Approximate mass/u
• Rutherford and his students (Hans Geiger and Ernest Marsden) bombarded very thin gold foil with α–particles
• Rutherford's scattering experiment


  1. A stream of high energy α–particles from a radioactive source was directed at a thin foil (thickness ∼ 100 nm) of gold metal

  2. The thin gold foil had a circular fluorescent zinc sulphide screen around it

  3. Whenever α–particles struck the screen, a tiny flash of light was produced at that point
• Most of the α–particles passed through the gold foil undeflected
• A small fraction of the α–particles was deflected by small angles
• A very few α–particles (∼1 in 20,000) bounced back, that is, were deflected by nearly 180°