Chemistry Timeline

Democritus: Developed concept of the Atom

370 BCE

Democritus believed that everything is composed of atoms and that they were uncuttable and are constantly moving which he explained correctly. His beliefs were based of reasoning and not science. He also said that they were indivisible which is true. Overall, his ideas led to the atom being discovered.

John Dalton: Atoms were responsible for the combinations of elements found in compounds.s

1808

In 1808, Dalton created his atomic theory which consisted of many different theories, one being that all matter is made up of tiny particles called atoms. His theory explained that the atom is the smallest particle that retains the properties of an element but incorrectly explained that atoms of the same element are not completely identical and that they consist of even smaller particles. He discovered this through experiments on different gases.

Eugen Goldstein: Discovered the existence of positively charged rays which were known as anode rays.

1886

In 1886, Goldstein discovered the proton through his observations of discharge tubes where he saw rays. He called these canal rays.His experiments explained that these rays would knock electrons and attract them to a positively charged electrode.

J. J. Thomson: Particles (Electrons) in Cathode Rays must be negatively charged.

1897

In 1897, Thomson, through his experiments of applying electricity to a glass tube, produced rays of small particles. In the future these would be known as electrons. His discoveries explained that these particles were negatively charged. His theory did not explain how the atoms were neutral, leading to the discovery of protons. It also didn't explain that electrons were smaller than atoms and they had small masses.

Robert A. Millikan: Discovered the elementary charge which was the electric charge carried by one electron

1909

In 1909, Millikan performed the oil drip experiment which consisted of charging two drops of oil between electrodes to determine their charge. This discovery explained the size of charge on an electron and that there was a smallest unit of charge.

Ernest Rutherford: Protons must be contained in a small, positively charged region at the center of the atom.

1911

In an experiment with Thomson, Rutherford aimed positively charged particles at a thin piece of gold foil to test the Thomson model. He observed that particles were deflected as they passed through the foil, which contrasted the Thomson Model. This explained that the positively charged region at the center of the atom and that the electrons were in the space around the center

James Chadwick:Proved the existence of neutrons or uncharged atoms

1932

In 1932, proved that neutrons existed through his experiment. In his experiment he used kinematics and determined the velocity of protons. From then he could find the mass mass of the neutral radiation. This discovery explained that atoms consisted of a neutral particle that had about the same mass as a proton. It did not explain