Atomic theory timeline

Aristotle 350BC

350

He proposed that all matter consisted of four elements including Earth, Air, Water and Fire. There were also four qualities including dryness, hotness, coldness, and moistness.

Democritus 450BC

450

He brought up the idea of atom. He proposed that matter was composed of tiny particles, atoms. They join with other atoms to create more atoms.

Alchemists 500BC

500

First people to test the theories that had emerged about the existence of atoms and the different elements. Before the age of the Alchemists, the concept of atomic theory was just a bunch of ideas.

Leucippus 500BC

500

One of the first greeks to develop the theory of the atom, which was formalized and elaborated on by his pupil Democritus

Isaac Newton

Apr 23, 1704

Proposed a universe with small solid masses in motion.

Benjamin Franklin

Apr 23, 1752

He introduced the theory that contained the first clear statement of the law of conservation of electric charge.

Antoine Lavoisier

Apr 23, 1789

He was the one to formulate the law of conservation of mass, which states that the total mass in a chemical reaction remains constant.

John Dalton

Apr 23, 1803

He proposed that all matter is made of atoms and they are indivisible and indestructible, all atoms of a given element are identical in mass and properties, compounds are formed by a combination of two or more different kinds of atoms, and a chemical reaction is a rearrangement of atoms.

John Dalton

Apr 23, 1803

His atomic model

Joseph Proust

Apr 23, 1812

He published his law of definite proportions in 1794 (accepted in 1812).

Michael Faraday

Apr 23, 1832

He suggested that the structure of atoms was related to electricity in someway. He developed laws of electrolysis.

J. Plucker

Apr 23, 1859

Built the first Cathode Ray Tube

Dmitri Mendeleev

Apr 23, 1869

He created the first formal periodic table of element with the 56 elements that were known during his time.

Henri Becquerel

Apr 23, 1896

He discovered radioactivity and was able to isolate radium and polonium.

Joseph John Thomson

Apr 23, 1897

He did cathode ray experiment and discovered the negatively charged particle, electron.

Ernest rutherford

Apr 23, 1898

He did gold foil experiment and discovered the center of the atom, nucleus.

Marie nd Pierre Curie

Apr 23, 1898

By studying the decay process of Uranium and Thorium, she discovered radium and polonium and invented the term ‘radioactivity’.

Ritherford

Apr 23, 1898

Gold foil experiment

Max Planck

Apr 23, 1900

He used the idea of quanta (discrete units of energy) to explain hot glowing matter.

William Crookes

Apr 23, 1903

Studied the then new concept of radioactivity. He achieved the separation of uranium from uranium-x. He then observed the gradual decay of the product.

Albert Einstein

Apr 23, 1905

Published a paper that proved beyond doubt the existence of atoms using the movement of pollen particles in still water, etc.

Hans Geiger

Apr 23, 1906

Produced device that made a noise when hit with alpha particles

Robert Millikan

Apr 23, 1909

He did oil drop experiment and determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 grams) of an electron.

Milikan

Apr 23, 1909

oil drop experiment

Niels Bohr

Apr 23, 1913

He developed the Bohr atomic model, with electrons travelling in orbits around the nucleus, and chemical properties being determined by how many electrons are in the outer orbits. He also integrated the Planck quantum theory, stating that when electrons change orbits, they emit a quantum of discrete energy.

Henry Moseley

Apr 23, 1913

discovered a systematic mathematical relationship between the wavelengths of the X-rays produced and the atomic numbers of the metals that were used

Louis deBroglie

Apr 23, 1924

He introduced (as his doctoral thesis) his theory of particle-wave duality. This theory suggests that particles can behave like waves, and that waves can behave like particles.

Erwin Schrodinger

Apr 23, 1926

He made the modern accepted model of the atom, with orbitals rather than the shells/energy levels that Bohr described.

Werner Heinsenberg

Apr 23, 1927

He introduced the uncertainty principle. A principle which states that the speed and location of an electron cannot be determined simultaneously.

James Chadwick

Apr 23, 1932

He established that atomic number is determined by the numbers of protons in an atom.
He also discovered the fourth subatomic particle,the neutron.