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400 BCE
Democritus
Democritus, also known as the "laughing philosopher", was a Greek philosopher who is famous for discovering that all material is made up of tiny atoms and is one of the two main founders of the atomic theory. -
387 BCE
Plato
Plato was a Greek philosopher born in Athens. He founded the Academy, which is considered by some to be the first university. He also had many important philosophical ideas. -
300 BCE
Aristotle
Aristotle, a Greek philosopher, did not believe in the atomic theory. Instead, he believed that all substances and materials found on Earth were made up of fire, earth, water, and air instead of atoms. He also believed that there were four main qualities to all substances: cold, hot, wet, and dry. -
500
The Alchemists
They performed alchemy, or manipulating matter to create something else by changing their properties. This would go on to form a basic foundation for modern science and chemistry. -
Lavoisier
Antoine Lavoisier was a French chemist who discovered oxygen's role in respiration and combustion, disproving the common theory of combustion at the time. He also helped create the metric system and helped improve chemical nomenclature. -
John Dalton
John Dalton was an English physicist, chemist, and meteorologist who introduced the atomic theory in 1808. This was the first attempt to describe all matter to be composed of tiny, invisible particles called atoms. -
Newland's Law of Octaves
In 1864, John Newland, an English scientist, came up with the law of octaves which showed that every eighth element displayed similar properties when arranged according to increasing atomic weight. -
Mendeleev's Pd. Table
Dmitri Mendeleev was a Russian chemist who developed the periodic table in 1869, leaving some blank spots for not yet discovered elements. -
Photoelectric Effect
In 1887, German physicist Heinrich Rudolf Hertz discovered the photoelectric effect where a material releases electrically-charged particles when the material absorbs electromagnetic radiation. -
Discovery of Radioactivity
In March of 1896, Henri Becquerel discovered radioactivity on accident when he was investigating the newly-discovered X-ray with naturally fluorescent materials. -
Discovery of the Electron
In October of 1897, J.J. Thomson discovered the electron by experimenting with a cathode ray tube. He also came up with the plum-pudding atom model where negatively-charged atoms were found within a positively-charged soup-like substance. -
Planck's Quantum Theory of Light
In 1900, Planck came up with a theory stating that different molecules and atoms can absorb or emit energy only in separate ways. The smallest amount of energy is called quantum. -
Plum Pudding Model
The plum pudding model is a historical model of an atom, displaying positive charge surrounding electrons. -
Charge of the Electron
Robert Millikan, an American physicist and chemist at the University of Chicago, used an oil-drop method to discover the calculation of the charge carried by a single electron. -
Robert Millikan
Robert Millikan, an American physicist and chemist at the University of Chicago, used an oil-drop method to discover the calculation of the charge carried by a single electron. -
Rutherford's Gold Foil Experiment
In 1911, Rutherford conducted an experiment where a beam of alpha particles hit a piece of gold foil. Most of the particles passed right through, however a few were projected back in the opposite direction, showing that the atom was mostly empty space surrounding the central nucleus. This was a new discovery because previously atoms were thought of as a plum-pudding model. -
Bohr's Planetary Model
The planetary model, created by Niels Bohr in 1913, is a model where electrons orbit around a central nucleus in the atom. -
Mosley's Atomic Number
Mosley discovered the atomic number is the amount of positive charges found in the nucleus. He also discovered that there were three not yet known elements between aluminum and gold. They had the atomic numbers of 61, 43, and 75. -
Discovery of the Proton
In the early 1900's, Ernest Rutherford conducted an experiment resulting in a nuclear reaction and the first splitting of an atom. This led to the discovery of the proton, a positively-charged atom. -
Schrodinger Equation
It is a linear equation from 1926 created by Schrodinger that predicts the behavior of a dynamic system and is used in quantum mechanics. -
Heisenberg Uncertainty Principle
In 1927, Werner Heisenberg came up with the Uncertainty Principle, stating that it is impossible to accurately know both the speed and position of a particle as the more that is known about one aspect, the less is known about the other. -
Discovery of the Neutron
In May 1932, James Chadwick discovered a new uncharged particle found in the core which he named the neutron. He found it through an experiment he conducted where Beryllium was hit by alpha particles. The radiation went through a lead shield which could not be explained by the known particles at the time.
