Atomic Theory Timeline

Main

Democritus

0400 bc

Democritus was the first scientist to suggest that all matter was composed of small, indivisible particles and that the properties of matter was determined by the properties of these pieces of matter.

John Dalton

1803

Dalton was the first scientist to theorize that atoms of different elements had different weights and proposed a number of ideas about the atom that remains true today.

Eugen Goldstein

1886

Because of his Perforated Cathode Ray experiment, Goldstein concluded that atoms had a positively charged particle, the proton, because they flew through the holes in the negative cathode. He is also credited with the discovery of canal rays.

J.J. Thompson

1897

Thomson discovered the electron through a series of experiments. He concluded that electrons were much smaller than the actual atom and the charge to mass ratio was very large. Thomson also did experiments with cathode rays.

Max Planck

1900

Max Planck was responsible for the discovery of the quantum theory, stating that energy must be released in small quantities rather than in a continuous wave and explained atomic emission spectra through the quantum theory.

Albert Einstein

1905

Einstein argued that light energy was released in small packets called quanta. He was also the one to discover the quality of mass and energy, e=mc2.

Robert Millikan

1909

Millikan used his Oil Drop Experiment to prove the charge and mass of an electron. He also concluded that changes in energy occurred in tiny increments, proving the Quantum Theory.

Ernest Rutherford

1911

Rutherford theorized that an atom had a very dense, positively charged core, due to particles being deflected by a sheet of gold foil, and that negatively charged electrons orbited the nucleus like planets around the sun.

Henry Moseley

1913

In contrast to Mendeleev's periodic table, Moseley ordered his table by atomic number and wavelengths of x-ray spectral lines rather than atomic weight.

Neils Bohr

1913

Bohr applied quantum theory to Rutherford's atomic structure involving orbiting electrons. Bohr concluded that electrons traveled in stationary orbits, but this also led to the discovery of energy levels and that there is a limited number of electron energies allowed.

Louis de Broglie

1921

de Broglie was the scientist to introduce the theory of wave/particle duality, suggesting that particles act like waves and that waves act like particles. This was described by the equation λ=h/p, where λ is wavelength, h is Planck's constant, and p is momentum.

Werner Heisenberg

1923

Heisenberg discovered that knowledge of the exact location and momentum of particles at the atomic level wasn't possible. Observation required light, and the energy from the light would alter the position and momentum of whatever was being observed.

Erwin Schrodinger

1926

Schrodinger, known for his quantum mechanical model, took the theories and ideas of other scientists before him and put them together to come up with his own equation. This equation proved that energy was quantized and that orbitals were essential to electron location. This equation explained chemical properties and reactivity of elements.

James Chadwick

1932

In 1932, James Chadwick discovered the neutron, a neutrally charged particle in the nucleus. His discovery ultimately lead to the fission of uranium-235 and the making of the atomic bomb.

Other Scientists

Joseph Priestly

1774

Discovered oxygen, carbon monoxide, and nitrogen dioxide

Antoine Lavoisier

1785

Considered the father of modern chemistry due to his discovery of the law of conservation of matter.

Alessandro Volta

1799

Invented the world's first battery.

Louis Pasteur

1857

Considered one of the saviors of mankind because of the importance of Pasteur's discovery of the pasteurization process.

Alfred Noble - Dynamite

1863

Guglielmo Marconi

1895

First to transmit and receive a radio wave.

Henri Becquerel

1896

Discovered radiation of uranium

Alfred Noble - Noble Prize

1901

Much of his will went to establishing the Nobel Prize.

Marie Curie

1902

Isolated radium

Sir Francis Banting

1922

First to isolate insulin

Philo Farnsworth

1927

Invented the television

Michael Faraday

1931

Created the first electric generator

Wallace Carothers - Neoprene

1931

Leo Szilard

1934

Invented the nuclear chain reaction and the atomic bomb

Wallace Carothers - Nylon

1935

Kary Mullis

1985

Discovered the polymerase chain reaction

Historical Developments

Gutenberg Printing Press

1440

Discovery of Americas

1492

Protestant Reformation

1517

Galileo's conviction of heresy

1633

Declaration of Independence

1776

Civil War

1861 - 1865

Tanscontinental Railroad

1869

Assassination of Archduke Ferdinand

1914

Bolshevik Revolution

1917

Treaty of Versailles

1919

Peral Harbor

1941

Initial concept of the internet

1961

First cell phone

1973