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400 BCE
Democritus
He was a great mathematician and philosopher. He and his master were the firs propose the existence of atoms which in their words are:
Eternal, indivisible, homogeneous, invisible.
Atom are different in shape and size but they have the same internal qualities.
Matter properties only differ based on how they join. -
Dalton
He developed a very consistent theory based on his research works, his theory include five main points:
Elements are made of small particles called atoms.
Atoms of a given element are identical in size, mass and other properties, atoms of different elements do not.
Atoms cannot be subdivided, created or destroyed.
Atoms of identical or different elements combine in a simple whole-number ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated or rearranged. -
J.J. Thomson
Thomson is credited with the discovery and identification of the electron, and the discovery of the first subatomic particle. He used a cathodic tube of rays to investigate the atom. His outcomes were used to develop the PLUM PUDDING model which consists of a mass electrically neutral and in its surface are the electrons which are negatively charged. -
Robert Milikan
Scientist to whom the Oil Drop Experiment is credited to.Through this experiment accurately determined the charge of an electron (e =1 .6x10^-19 coulombs). He later determined the mass of an electron as well (9.11x10^-28 grams). He was very interested in the finding of the electron by JJ Thomson which led him to predict that the atom was 1000 times smaller than the atom. Was involved in Quantum Theory inspired by Max Planck. -
Ernest Rutherford
Rutherford, New Zealand-British physicist, performed Gold-Foil experiment in order to demonstrate the inaccurate model Thomson had depicted. With the results of his experiment developed the Rutherford model. This new model described an atom with a nucleus containing most of the atom's mass and the electrons orbiting the nucleus. Through this experiment, he came to the conclusion that the atom contains mostly empty space. -
Niels Bohr
Bohr, Danish physicist, came up with his own model of the atom based off of Rutherford's model, the Bohr model. This model adapts Rutherford's nuclear structure to Max Plank's Quantum Theory. Through his model introduced the idea of energy levels in the atom and that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a quantum of discrete energy. -
Erwin Schrödinger
He took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud. -
James Chadwick
He bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists..
