Timeline of Atomic Theory

Aristotle

322

Aristotle believed in the four elements of air, earth, water and fire. Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter.

Democritus

460

The theory of Democritus and Leucippus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Of the mass of atoms, Democritus said "The more any indivisible exceeds, the heavier it is."

Phlogiston Theory

Mar 6, 1667

Johann Becher and Georg Stahl developed the Phlogiston theory. Basically, when something burned, it lost phlogiston to the air. Yet, a problem with the theory was that burning of metals resulted in an increase in the mass. This problem was solved by assigning negative mass to phlogiston.

Discovery of Oxygen

Mar 6, 1774

Joseph Priestly conducted an experiment, focusing sunlight on mercuric oxide (HgO) inside a glass tube, which liberated a gas he named "dephlogisticated air". He noted that candles burned brighter in the gas and that a mouse was more active and lived longer while breathing it.

Law of Conservation of Mass

Mar 7, 1789

Antoine Lavoisier (1743-1794) proposed the Combustion Theory which was based on sound mass measurements. He also proposed the Law of Conversation of Mass which means that mass is neither created nor destroyed in chemical reactions. Or, that the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction.

Law of Constant Composition

Mar 7, 1799

Joseph Proust (1754-1826) proposes the Law of Constant Composition, which states samples of a pure compound always contain the same elements in the same mass proportion.

Law of Multiple Proportions

Mar 7, 1803

John Dalton (1766-1844) proposed the Law of Multiple Proportions, leading directly to the proposal of the Atomic Theory. Dalton recognized the existence of atoms of elements and that compounds formed from the union of these atoms. He therefore assumed that simplest ratios would be used in nature and came up with a formula for water of HO.

Law of Combining Volumes

Mar 7, 1808

Joseph Gay-Lussac (1778-1850) proposed the Law of Combining Volumes in 1808. He showed that at the same temperature and pressure, two volumes of hydrogen gas reacted with one volume of oxygen gas to produce two volumes of water (as a gas).

Avogadro's Hypothesis

Mar 7, 1811

Amadeo Avogadro (1776-1856) proposed what is now known as Avogadro's Hypothesis in 1811. The hypothesis states that at the same temperature and pressure, equal volumes of gases contain the same number of molecules or atoms. When this is combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2 and H2O, respectively.

Stanislao Cannizzaro

Mar 7, 1860

Cannizzaro showed how the atomic weights of elements contained in volatile compounds can be deduced from the molecular weights of those compounds, and how the atomic weights of elements of whose compounds the vapour densities are unknown can be determined from a knowledge of their specific heats.

Proposal of the Periodic Table

Mar 7, 1869

Dimitri Mendeleev (1834-1907) proposed the periodic law and developed the first periodic table in 1869. Medeleev's table was arranged according to increasing atomic weight and left holes for elements that were yet to be discovered.

Cathode Ray Tube

Mar 7, 1897

J. J. Thomson (1856-1940) identified the negatively charged electron in the cathode ray tube in 1897. He deduced that the electron was a component of all matter and calculated the charge to mass ratio for the electron.
Thomson and others also studied the positive rays in the cathode ray tube and discovered that the charge to mass ratio depended on filling gas in the tube.

Robert Milikan

Mar 7, 1909

Robert Millikan (1868-1953) determined the unit charge of the electron in 1909 with his oil drop experiment at the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms.

Proposal of the Nuclear Atom

Mar 7, 1911

Ernst Rutherford (1871-1937) proposed the nuclear atom as the result of the gold-foil experiment in 1911. Rutherford proposed that all of the positive charge and all of the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons.

Henry Moseley

Mar 7, 1913

Henry Moseley (1887-1915) discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table. In 1913, he proposed that the relationship was a function of the positive charge on the nucleus. This rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements. This new table left additional holes for elements that would soon be discovered.

Proposal of the Neutron

Mar 7, 1920

Although positive particles had been discussed for some time, it was Ernest Rutherford in 1920 that first referred to the hydrogen nucleus as a proton. Also in 1920, Rutherford proposed the existence of the third atomic particle, the neutron.