History of an Atom timeline.

Democritus was one of the first atomic theorists in the fifth century B.C. During this time period, he came up with the reasoning that if an object was divided over and over again, at some point there would be pieces that are indivisible. He called these small pieces of matter "atomos," which is the Greek world for indivisible. This led to Democritus's atomic theory. The theory stated, "The universe is composed of two elements: the atoms and the void in which they exist and move."

John Dalton came up with the atomic theory of matter in 1803. Along with this theory, he produced many ideas to support it. For example, all elements are made up of atoms, atoms of different elements are different, and atoms of different elements combine to form a compound. To achieve his theory, Dalton created a chart of atomic weights, one of the first of its kind. It was called, 'A New System of Chemical Philosophy'.

J.J. Thomson was the man who discovered the electron. In 1897, Thomson performed a series of experiments to study electric charges. The experiment that lead to his discovery of the electron were gas discharge tubes. He noticed the movement of cathode rays in the tube. The rays moved from the negative end of the tube to the positive end. He realized that the rays were made out of negatively charged particles, electrons.

Lord Ernest Rutherford conducted his famous experiment called the Gold Foil Experiment in 1911. During this experiment, alpha particles were shot at gold foil. Most particles passed through the foil, while some particles went straight back or deflected like they had hit something. In this, he proved that atoms are made of a small, positive nucleus and that a positive nucleus repels positive alpha particles. Also, atoms are mostly empty space.

Niels Bohr proposed a model of an atom, similar to a solar system in 1913. The electrons orbit the nucleus like planets do the sun. With this model, he stated that the electrons have a certain distance from the nucleus, giving them different levels of energy based on their location. Level 1 holding the least amount of electrons, 2. To level 4 holding the most, 32 electrons. He concluded that when electrons lose energy they go down a level. When electrons gain energy, they move to a higher level.