Democritus announces that the atom is the simplest unit of all matter.
Approx. 430 BC
Leucippus is considered to be the founder of the theory that the universe consists of two different elements, which he called 'the full' and 'the empty'.
Aristotle decided that there were only 4 elements: fire, water, air, and earth. He declared that all matter was made up of these four elements.
In their quest to turn other substances into gold, alchemists developed many tools and techniques for working with chemicals. This includes equipment still used today, such as beakers, flasks, tongs, funnels, and the mortar and pestle.
Paracelsus argued that the body was a chemical system that had to be in harmony with the environment and balanced internally. He developed new ways of practicing medicine.
Boyle discovered that the volume of a gas decreases with increasing pressure and vice versa, which is today called Boyle's law.
Stahl developed the Phlogiston theory, which states that when something burned, it lost phlogiston to the air.
Joseph Priestly discovered oxygen and several other gases.
Lavosier discovered that oxygen was the part of air that combines with substances as they burn.
Proust published the Law of Definite Proportions, which states that a compound is composed of exact proportions of elements regardless of how the compound was created.
John Dalton publishes his Atomic Theory, stating that all matter is composed of small and invisible atoms.
Joseph Louis Gay-Lussac
Gay-Lussac's law of combining volumes stated that when two gases react, the volumes of the reactants and products are whole number ratios.
Avogadro discovered that the relative molecular weights of any two gases are the same as the ratio of the densities of the two gases under the same conditions of temperature and pressure.
Berzelius established the law of constant proportions, which stated that the elements in inorganic substances are bound together in definite proportions by weight.
William Crookes discovers Cathode Rays, which are beams of electrons emitted from the cathode of a high vacuum tube.
Becquerel discovered a property of a Pitchblend compound, which was that it gave off a fluorescent glow with or without sunshine.
JJ Thomson discovered that all atoms have a negative charge, and he renamed cathode rays electrons.
Robert Millikan discovered that the mass of an electron was 9.11E-28 grams.
Rutherford, first having discovered that there are three different types of radioactivity, created a new atom model. He believed that an atom contained a tiny, positively charged nucleus, and that the nucleus was surrounded by high speeding electrons.
Mosely discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each element on the periodic table. He believed this to be because of a positive charge in the nucleus.
Bohr developed a new model of the atom, which proposed that electrons are arranged in concentric circular orbits around the nucleus.
Planck originated the quantum theory of energy, which helped understand atomic and subatomic processes.
Heisenberg created matrix mechanics, the first version of quantum mechanics. He calculated the behavior of electrons.
Formulated the Pauli principle, which states that no two electrons in an atom could have identical sets of quantum numbers.
Schrodinger, using the Bohr model of an atom and mathematical equations, discovered a way to calculate the likelihood of an electron being in a certain position in an atom.
Hund discovered quantum tunneling and created Hund's rule of maximum multiplicity.
Carl Wilhelm Scheele
Scheele discovered that air was a mixture of oxygen and nitrogen.