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Democritus develops the atomic theory of the universe.
Democritus adopted the theory originated by his teacher, Leucippus. He created the first model of an atom, a circle with no protons, neutrons, or electrons. This helped the overall understanding of the atom for scientists. -
Lavoisier finds explanation of combustion and respiration.
Lavoisier revolutionized chemistry. He named oxygen, carbon, and hydrogen, He also discovered that oxygen is involved in both combustion and respiration. -
Dalton develops the theory of atomism.
Dalton's theory is still used today in modern chemistry. It was:
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
John also visualized the atom as a sphere. -
Becquerel discovers evidence of radiation.
Doing experiments with uranium salts, Becquerel discovers naturally occurring radiation. -
J.J. Thomson discovers the electron
Thomson discovered the electron and created the Plum Pudding model. His discovery contributed to the understanding of the atom -
The Curies name polonium and radium.
After experimenting and studying Becquerel's experiments, Marie and Pierre Curie discover and name two new metals, polonium and radium. -
Rutherford confirms the existence of the nucleus and proton.
In his landmark Gold Foil Experiment, Rutherford discovers a positively charged region that we now know as the nucleus. -
Robert Milliken completes Oil Drop experiment.
In 1909, Milliken created an Oil Drop experiment that helped quantify the charge of an electron, which helped out understanding of the atom and the atomic theory. -
Bohr improves on the atomic theory.
In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. -
Moseley improves upon the Periodic Table of Elements.
Henry Moseley used X-ray to study atomic structure and his discoveries resulted in a more accurate positioning of elements in the Periodic Table. -
Louis de Broglie discovers wave nature of electrons.
De Broglie's ideas eventually lead to the creation of the wave mechanics theory which gave us more knowledge of physical nature. -
Pauli discovers the Pauli exclusion principle.
In 1925, Pauli discovers that no two electrons can share the same quantum state at the same time. -
James Chadwick discovers the neutron.
In an experiment, Chadwick discovers small neutrally charged particles, which we know today as neutrons.
