History of Atomic Theory

By: Alyssa McClendon



400 B.C.

Democritus was the first man to discover the idea of the atom. He called the first atom "atomos" which means not to be cut. He based his theory off observations and natural curiosity.

John Dalton


Proposed the Law of Multiple Proportions which led to the Atomic Theory. The theory contained five principles:
1) all matter is composed of atoms
2)atoms of given element are identical in properties
3)atoms of different elements differ in properties
4)atoms of different elements combine in a simple, whole number ratio to form compounds
5)in chemical reactions, atoms are combined, separated, rearranged, but never created, changed, or destroyed.
He did many experiments with gases which was measure the amount of water vapor that air could absorb at different temperatures.

J. J. Thomson


Thomson discovered the negative charged electrons and neutrons. He used a cathode ray tube to find the electrons outside of the atom. Also he used the cathode ray tube to study the protons. Thomson proposed the "Plum Pudding" model. In the model, the volume of the atom is composed of the protons. The smaller electrons are dispersed throughout the positive mass to maintain the neutral charge known as neutrons.

Robert Millikan


Millikan determined the unit charge of the electrons by the experiments he did. The main experiment was the oil drop which he put a charge on a small drop of oil. Then he measured how strong the electric field had to be to stop the oil from dropping. https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcQjR6WoEEWZ3lywqOgC57Zv6coP1VL5fAGeTn6PVk8tHwCvZt84Ww

Ernest Rutherford


Rutherford discovered that the nucleus and atoms were mostly made up of empty space. He performed the Gold Foil experiment. The experiment was that he shot positive particles at the gold foil which was 2000 atoms thick. Some of the particles shot back, some passed through, but others passed through on a diagonal.

Niels Bohr


Bohr came when Rutherford could not explain why electrons would not go to the nucleus. He created a model called the "Electron Cloud" that put electrons in energy levels. Closer to the nucleus was low energy and far away from the nucleus was high energy. The electrons could only be in one electron level. The electrons do not orbit but they bounce around in a cloud. He also created the "Wave Model" which proved it was impossible to find a particular position of an electron. He used the liquid drop for his experiment which was the neutrons and protons behave like molecules in a drop of water.

Werner Heisenberg


Werner contributed to the Atomic Theory by interpreting the behavior of elementary particles and atoms. He discovered the Uncertainty Principle which means electrons do not travel in neat orbits.

Erwin Schrödinger


Schrödinger took Bohr's model one step further by using mathematical equations to find the possibility of finding an electron in a certain position. He created the quantum mechanical model of the atom which does not create the exact path the electron will take but will predict its location. https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcR6CRW2p1r-z-kl1DgP7ft_qC-vbVGKPF0YZLGWPB_mN7RoFVj3 https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcS_v_dIROHP9s7iutGZW392QBxTq7ALO5ma0rtiaIQdxL50v_txtQ

James Chadwick


Chadwick contributed to the understanding of an atom by discovering the neutron. James Chadwick bombarded beryllium atoms with alpha particles which created a radiation. The radiation is composed of a neutral charge and mass of a proton.