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400 BCE
Democritus
Democritus hypothesized that if the stone was cut into smaller and smaller pieces eventually it would become so small it would be indivisible. The methods to verify this theory did not exist in that era. -
John Dalton
Dalton based his theory on the law of conservation of mass/the law of constant composition. He conducted “experiments with gases," developing five major principles; all matter is made of atoms (which are indivisible), all atoms of a given element are identical in mass and properties, compounds are combinations of two or more different types of atoms, a chemical reaction is a rearrangement of atoms, and atoms cannot be created or destroyed. -
JJ Thompson
Thompson discovered the charge of the atom and accounted for it with negatively charged electrons within the overall positive charge. He realized the accepted model of an atom did not account for negatively or positively charged particles and by experimenting with a cathode ray tube he demonstrated that cathode rays were negatively charged. This led him to his proposed plum pudding model of the atom (raisins representing negative electrons, and the dough containing an overall positive charge). -
Ernest Rutherford
Rutherford conducted the Gold Foil experiment where he shot alpha particles at a thin sheet of gold. He expected all the alpha particles to go straight through the foil (following plum pudding model) but this was not the case; some of the atoms bounced off. Based on his observations that most particles went through but a few rebounded, he concluded there must be one large mass in the center of an atom. His new nuclear model was mostly empty space with a small dense positively-charged nucleus. -
Neils Bohr
Because Rutherford’s model did not explain the chemical properties of elements, for example, why metals or compounds give of unique colors when heated, Bohr developed his own model. The main ideas of this model of the atom were that there were fixed orbitals for electrons to follow around the nucleus and that electrons can only have certain energy values. -
Erwin Schrödinger
Although Bohr’s model worked very well for the hydrogen atom however it did not explain the behaviors of atoms with more than 1 electron. In response to this inconsistency Schrödinger developed the Quantum Mechanical Model, in which electron clouds describe the probability of where an electron might be found and electrons can only have certain energy values.
