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400 BCE
The Existence of Atoms - Democritus
Democritus theorised that all forms of matter could be broken down until only small particles remained. He named these particles 'atomos', meaning 'indivisible' or 'uncuttable'. He also theorised that atoms of the same element are the same, and compounds are formed by various types of atoms. 2/3 of his theory was correct, however atoms are not indivisible, for they are made of subatomic particles (protons, neutrons and electrons). They are however, the basic building blocks of matter. -
Period: 400 BCE to
Period of Atomic Model Development
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Atomic Relation to Elemental Properties - John Dalton
Dalton's discoveries of the different amounts of oxygen for the same amount of carbon resulted in his theory that pure elements were made from atoms of equal mass, whereas different elements had atoms of varying types and masses. The relation to an atom's mass does indeed play a crucial role in determining its elemental type, as well as its position in the periodic table. At this point in time, Dalton surmised that atoms were solid spheres, which is not true. -
'Plum Pudding' Model - J.J.Thompson
J.J. Thompson came up with the theory that electrons were evenly but also randomly spaced in a positively charged sphere that gave it its apt label, the 'Plum Pudding' model. His conclusions were drawn based on experiments involving cathode ray tubes, which showed that negatively charged particles existed within atoms. He named what we now know as 'electrons' as 'corpuscles'. He essentially was the first to discover the presence of electrons. -
Nucleus and Electrons - E. Rutherford
Rutherford's complex experiment comprised of alpha rays being directed at a thin sheet of gold foil. Most of the alpha particles did not transmit through the foil and deflected at large angles. This allowed Rutherford to conclude that majority of the atom was occupied by empty space, with a positively charged nucleus in the center and electrons forming the outermost layer. A cloud of electrons (taking most of an atom's mass) does in fact surround a nucleus with a positive charge. -
Electron Pathways - Niels Bohr
Bohr's atomic model described the stable orbitals of electrons around the positively charged nucleus branching off the concentric structure established by Rutherford. By doing this, he created a planetary model which emulated the features of the solar system. He established that electrons travelled along pathways called orbitals which had associated energy levels, which were assigned letters. This explained why different elements produced different colours depending on the position of electrons. -
Motion of Electrons - E. Shrodinger
Shrodinger's most prominent attribution to the development of the atomic model was the formulation of the Shrondinger wave equation which was a mathematical formula that could be used to describe the movement of electrons within an atom. This was also used to calculate the energy levels of waves. He did this by first assuming that electrons could be regarded as small particles and waves. He is also accredited for a thought experiment known as the Shrodinger's Cat. -
Neutrons - James Chadwick
James Chadwick proved the existence of neutrons (subatomic particles with neither a positive or negative charge) through a boron and alpha particle bombardment experiment, which led him to believe that there was a particle with mass but no charge. By measuring the other particles of collision, he discovered that neutrons were particles of approximate mass to protons, but without the positive charge. This ultimately supported the known of the overall neutral charge of atoms.