Timeline: History of The Atomic Theory

400 BCE

Democritus

Democritus was the first to introduce the Atomic Theory stating that all matter is made up of small indestructible units called atoms.

Period: 400 BCE to 1942

Atom Theory

Timeline

1704

Isaac Newton

Isaac Newton theorized theorized a mechanical universe with small, solid masses in motion.
The first law states that every object in the state of uniform motion tends to remain in the same state of motion unless an external force is applied to it
The second law states that the net force of an object is proportional to the time rate of change to its linear momentum.
The third law states that for every action there is an equal and opposite reaction.

1803

John Dalton

Proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.

1832

Michael Faraday

Michael Faraday developed the two laws of electrochemistry.
m=(Q/F)(M/z)
m is the mass of the substance liberated at an electrode in gms
Q is the total electric charge passed through the substance in coulombs
F = 96500 C mol−1 is the Faraday constant
M is the molar mass of the substance in grams per mol
z is the valence number of ions of the substance
n=(It/F)(1/z)
n is the amount of substance ("number of moles") liberated: n = m/M
t is the total time the constant current was applied.

1859

J. Plucker

J. Plucker built one of the first cathode-ray tubes.

1869

Dmitri Meneleev

Dmitri Mendeleev created the periodic table. He formulated the Periodic Law, created a farsighted version of the periodic table of elements, and used it to correct the properties of some already discovered elements and also to predict the properties of eight elements yet to be discovered. He arranged the periodic table in order from smallest to largest mass size.

1873

James Clerk Maxwell

James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.

1874

G.J. Stoney

G.J. Stoney theorized that electricity was comprised of negative particles he called electrons

1886

E. Goldstein

E. Goldstein discovered canal rays, which have a positive charge equal to an electron.

1895

Wilhelm Roentgen

Wilhelm Roentgen discovered x-rays

1897

J.J. Thomson

J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged.

1898

Ernest Rutherford

Rutherford discovered alpha, beta, and gamma rays in radiation in his gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus.

1898

Maie Sklodowska Curie

Marie Sklodowska Curie discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium

1900

Max Planck

Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.

1900

Frederick Soddy

Frederick Soddy came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.

1903

Hantaro Nagaoka

Hantaro Nagaoka proposed an atomic model called the Saturnian Model to describe the structure of an atom.

1904

Richard Abegg

Richard Abegg found that inert gases have a “stable electron configuration.”

1906

Hans Geiger

Hans Geiger invented a device that could detect alpha particles.

1914

H.G.J. Moseley

H.G.J. Moseley discovered that the number of protons in an element determines its atomic number

1919

Francis William Aston

Francis William Aston used a mass spectrograph to identify 212 isotopes

1922

Niels Bohr

Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.

1923

Louis de Broglie

Louis de Broglie proposed that electrons have a wave/particle duality

1929

Cockcroft and Walton

Cockcroft / Walton created the first nuclear reaction, producing alpha particles

1930

Paul Dirac

Paul Dirac proposed the existence of anti-particles

1932

James Chadwick

James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. James Chadwick discovered neutrons, particles whose mass was close to that of a proton.

1938