01 Introduction to Chemistry
02 The Structure of Atoms
03 Concept of Mole, Formulae and Equations
04 Periodic Table of Elements
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2.2.1 The History of Development of the Atomic Model

History of Development of the Model of Atom

Table below shows the scientists that contribute to the development of the Model of Atom.
John Dalton
  • Five main points of Dalton’s Atomic Theory
  • All matter is composed of extremely small particles called atoms.
  • All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element.
  • Atoms cannot be created, divided into smaller particles, or destroyed.
  • Different atoms combine in simple whole-number ratios to form compounds.
  • In a chemical reaction, atoms are separated, combined, or rearranged.
  • Atoms consist of even smaller particles called electrons, protons and neutrons.
  • Atoms can be created and destroyed in the nuclear reactions such as nuclear fusion and nuclear fission..
  • Atoms of the same element can have different physical properties, for example, isotopes of hydrogen.
J.J. Thomson The electrons were positioned uniformly throughout the atom.
Ernest Rutherford
  • the atom is mostly empty space,
  • most of the atom’s mass concentrated in a tiny center, the nucleus and electrons being held in orbit around it by electrostatic attraction.
  • The nucleus was around 10-15 meters in diameter, in the centre of a 10-10 metre diameter atom.
  • Those alpha particles that had come into close proximity with the nucleus had been strongly deflected whereas the majority had passed at a relatively great distance to it.
Neils Bohr
  • Electrons in an atom of an element are not randomly distributed around the atomic nucleus.
  • Electrons move around the nucleus in fixed orbits.
  • Each orbit forms a circle and has a fixed distance from the nucleus.
James Chadwick
  • Chadwick discovered the presence of neutrons in the nucleus.
  • He concluded that the nucleus contains another tiny particle known as a neutron that has no charge.
  • The neutron mass is almost similar to the proton mass.
  • All nuclei contain protons and neutrons, except for the hydrogen which contains protons. only

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