Atomic Theory

Events

Alchemists

500 B.C.

The Alchemists were able to break down the chemical compositions of what they thought were elements (wind, fire, water). This would eventually lead to the creation of the periodic table.

Leucippus

450 B.C.

Originated the theory of atomism, which is the theory that everything is made of atoms.

Democritus

450 B.C.

Proposed that matter could not be infinitely divided, and that atoms were the smallest piece of matter.

Aristotle

340 B.C.

Did not believe in atoms. He thought that there were only five elements (earth, air, water, fire, and aether) and that all matter was either made of one of these elements, or a combination of these elements.

The Roman Empire

64 A.D. - 1453 A.D

The Dark Ages

500 A.D. - 1500 A.D.

Paracelsus

1530

Figured out that syphilis is caused by inhaling metal vapors, and small amounts of mercury could treat it.

Georg Stahl

1670

Developed the Phlogiston theory.

Robert Boyle

1688

Proved Aristotle's theory wrong. He proposed the idea that there are more than 5 elements, and that these elements cannot be separated into simple components.

Carl Wilhelm Scheele

1770

He discovered oxygen, but he wasn’t given credit for it.

Joseph Priestley

1774

He was credited for discovering oxygen.

Lavoisier

1774

Proposed the Law Of Conservation of Mass, and the Combustion Theory.

Joseph Proust

1794

Published his law of definite proportions.

John Dalton

1803

Proposed 4 basic ideas of atomic theory: all elements are made of atoms, atoms of the same element are alike, atoms of different elements are different, and compounds are made by forming two or more elements.

Joseph Gay Lussac

1808

He discovered 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.

Amedeo Avogadro

1811

Proposed the Avogadro Hypothesis, which states that gasses that are the same temperature, pressure, and volume have the same number of atoms.

Jöns Jacob Berzelius

1818

Had obtained the atomic weights of at least 45 elements, and he was the one to start making symbols for each element (for example, the symbol for carbon monoxide is CO)

Sir William Crookes

1875

Created the Crookes tube, which would eventually help J.J. Thomas and other scientists discover new information about the atom.

Henri Becquerel

1896

Discovered that atoms could be radioactive.

J.J. Thomson

1897

Created the “Plum Pudding” model, which is where positively charged substances surrounded the negatively charged electron. This is what he thought an atom was. He also discovered electrons.

Max Planck

1900

The originator of the quantum theory of energy.

Ernest Rutherford

1908

He proved the Plum Pudding Model wrong, and proposed that atoms had a positively charged nucleus and electrons were scattered between the nucleus and the atoms edge.

Robert Millikan

1913

His oil drop experiment helped quantify an electron’s charge, which resulted in a better understanding of atoms and their structure.

Henry Moseley

1913

Discovered that the periodic table is best organized by atomic number.

Niels Bohr

1913

Proposed that electrons travel in specific orbits (energy levels) around the nucleus.

Werner Heisenberg

1925

His theories of quantum mechanics and uncertainty led to the discovery of allotropic forms of hydrogen.

Wolfgang Pauli

1925

Discovered that no two electrons in an atom can exist in the same quantum state.

Erwin Schrödinger

1926

He developed the “Electron Cloud Model.” This model shows that a dense nucleus in the middle of an atom is surrounded by a cloud of electrons.

Friedrich Hund

1927

The first person to take notice of quantum tunneling. He also developed his Rule of Maximum Multiplicity.