The World of Atom (Part IX)

ReferenceA Logical Approach to Theoretical Physics



Chapter 43: Strange Results from -Particle Scattering Hans Geiger (1882 – 1945) Ernest Marsden (1889 – 1970)
On a Diffuse Reflection of the Alpha-Particles. Deflections occurred that were too big to be consistent with Thomson model of atom.

Chapter 44: The Nuclear Atom Ernest Rutherford (1871 – 1937)
The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom. The large deviations observed experimentally must have been caused by single direct collisions. The alpha particle approached as much as 3 x 10-10 cm close to a point of enormous force in the atom.

Chapter 45: Atomic Structure Niels Bohr (1885 – 1962)
On the Constitution of Atoms and Molecules. Planck’s discovery of the quantum of action and Einstein’s concept of the photon could be combined with Rutherford’s discovery of how  particles are scattered by atomic nuclei to derive a self-consistent planetary atomic model.

Chapter 46: The Quantum Theory is Tested James Franck (1882 – 1964) Gustav Hertz (1887 – 1975)
Collisions between Electrons and Mercury Vapor Molecules and the Ionization Potential of Such Molecules. An atom can take on energy from collisions only in discrete amounts.

Chapter 47: The discovery of Isotopes Frederick Soddy (1877 – 1956)
The Radio-Elements and the Periodic Law. The radioactive transformations produced atoms of the same chemical species but of different weights.

Chapter 48: The Positive Rays J. J. Thomson (1856 – 1940)
Rays of Positive Electricity. There are many different kinds of particles in the positive rays. Development of a method to obtain a mass spectrum.

Chapter 49: Transmutation of an Element Ernest Rutherford (1871 – 1937)
Collision of Alpha Particles with Light Atoms.Nuclei of atoms could be disrupted and changed into other nuclei.

Chapter 50: The Diversity of Atoms Francis William Aston (1877 – 1945)
Positive Rays and Isotopes. Development of mass spectrometers and the discovery of isotopes at the lighter end of the periodic table.



  1. The size of the atom is of the order of 10-8 cm.
  2. The size of the nucleus is of the order of 10-10 cm.
  3. The mass of the atom is concentrated in the “point” nucleus at the center.
  4. The volume of the atom is made up of electrons.
  5. Electrons are in a dynamic equilibrium in a plane around the nucleus.
  6. The nucleus of the atom is positively charged, while the electrons are negatively charged.
  7. The total negative charge of the electrons is equal to the positive charge of the nucleus.
  8. Electrons are in a state of perpetual motion within the atom (like the atoms are in a fluid).
  9. It takes energy to push the electron closer to the nucleus.
  10. A chemical element can have different atomic weights.



  1. The bound electron within the atom is same as the individual unbound electron outside the atom.
  2. Mass of the electron is negligibly small in comparison with that of the nucleus.
  3. The velocity of the electron is small compared to that of light.
  4. Electron, settled in a circular, stationary orbit around the nucleus, can be treated by ordinary mechanics.
  5. Passing of the systems between different stationary states cannot be treated by ordinary mechanics, but by emission of a homogenous radiation, per Planck’s theory.
  6. During the binding of the electron a homogenous radiation is emitted (equal to half the frequency of revolution of the electron in its final orbit).


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