Atomic Model Time Line

Democritus the Greek philosopher was the first to postulate the atomic theory around 442 B.C. He posited that all things were made of minute particles that were indivisible, invisible and indestructible. Democritus named theses particles the atom from the Greek word atomos, which in English translates to uncuttable. His model of the atom was limited by the research technology of his time and simply consisted of a round solid ball as he never knew about the nucleus or electrons.

In 1803, John Dalton put forward what he called the atomic theory of matter. In his model, Dalton further reinforced Democritus’ theory that all matter is made of atoms and that atoms are indivisible and indestructible. Included in Dalton’s model were the theories that all atoms of a particular element are identical in mass and properties, a chemical reaction is a rearrangement of atoms and that compounds are formed by a combination of two or more different kinds of atoms. 

The research that led to the discovery of the electron began with an attempt at explaining the inconsistent behavior of cathode rays under magnetic and electric fields. Hertz had previously shown that magnetic fields deflected these rays in the same way as they would a negatively electrified particle moving in the direction of the rays. In 1897 Joseph John Thomson discovered the first elementary particle. This was the result of his experiments which finally proved that cathode rays could be deflected by both electric and magnetic fields, a condition that was required if his theory that the cathode ray was composed of fast moving charged particles was to hold true. 

The discovery of the proton was the result of gradual research by three physicists, Thomson, Rutherford and Chadwick. Rutherford was however the first scientist to classify the proton but having based his work on research by Thomson. Rutherford discovered the nucleus in 1911 with Chadwick later discovering the neutron in 1932. These discoveries eventually led to the authoritative discovery of the proton when physicists proved that the nucleus contained positively charged elements.

Up until 1930, physicists had supposed that the fundamental particles were protons and electrons. In 1932, James Chadwick calculated the mass of the neutral particle from Compton scattering data. Chadwick was later able to prove by bombarding targets other than hydrogen that the neutral particle was not a photon. It is now known that atoms are composed of three types of particles; electrons, protons and neutrons. Electrons have a negative electrical charge, protons which are larger and heavier than electrons have a positive charge while neutrons have no electrical charge at all. 

Rutherford considered the possibility of the existence of the neutron in 1920 because of the disparity found between the atomic number and the atomic mass of an atom. Then in their experiments in 1930, Viktor Ambartsumian and Dmitri Ivanenko demonstrated that the atom could not be composed of protons and neutrons alone. The neutron was discovered By James Chadwick in 1932. In his experiments he proved that the gamma ray hypothesis was untenable explaining that the new radiation was composed of uncharged particles. The experiments he conducted were to later prove that there existed a third uncharged particle, the neutron in an atom. This discovery was to then explain the spin of the nitrogen-14 nucleus which had baffled many physicists.

Niels Bohr’s work can be said to have been the beginning of the atomic structure finalization. His revolutionary hypotheses in combination with classical mechanics enabled him to explain the atomic spectra. De Broglie’s work was also crucial in the atomic structure finalization with postulates on why only certain discrete energy levels are attainable, the derivation of the de Broglie equation and quantum physical models of the atom.