Atomic Theory Project

Alchemists

250

Performed various feats such as distilling acetic acid from vinegar and inventing the word alcohol. The main goal of alchemists was to turn lead into gold. 250-1727

Democritus of Adbera

450

Democritus created his atomic theory in 450 BC: All matter is composed of atomos, atoms of the same element are the same, atoms of different elements are different, charastics of a substance were determined by the shape of its atoms, sweet things were made of smooth atoms, bitter things were made of sharp atoms, and atoms remained unchanged but combined with other atoms to make objects. He also created the first atomic model in 400 BC.

Isaac Newton

Apr 20, 1704

Proposed a mechanical universe with small solid masses in motion.

Benjamin Franklin

Jun 19, 1752

Benjamin Franklin’s famous stormy kite flight in June of 1752 led him to develop the idea that matter had charges which he represented using plus(+) and minus (-).

Joseph Proust

Apr 19, 1794

He first published his Law of Definite Proportions (or Law of Constant Composition) in 1794.This law states that a compound is composed of exact proportions of elements by mass regardless of how the compound was created.

John Dalton

Apr 19, 1803

He performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different.

Michael Faraday

Apr 20, 1830

He observed that one of the elements of the dissolved compound accumulated on one electrode, and the other element was deposited on the opposite electrode. It was clear to Faraday that electrical forces were responsible for the joining of atoms in compounds.

J. Plucker

Apr 20, 1859

Built one of the first gas discharge tubes ("cathode ray tube").

Dmitri Mendeleev

Apr 19, 1869

Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law.

James Maxwell

Apr 20, 1873

Maxwell's formulation of electricity and magnetism was published in A Treatise on Electricity and Magnetism (1873), which included the formulas today known as the Maxwell equations. Maxwell also showed that these equation implicitly required the existence of electromagnetic waves traveling at the speed of light.

William Crookes

Apr 19, 1879

Discovered cathode rays had the following properties: travel in straight lines from the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by electric fields and magnets to suggest a negative charge; because pinwheels in their path to spin indicating they have mass.

E. Goldstein

Apr 20, 1886

Used a CRT to study "canal rays" which had electrical and magnetic properties opposite of an electron.

G.J. Stoney

Apr 20, 1894

Proposed that electricity was made of discrete negative particles he called electrons". (Link to info on electrons)

Wilhelm Roentgen

Apr 20, 1895

Using a CRT he observed that nearby chemicals glowed. Further experiments found very penetrating rays coming from the CRT that were not deflected by a magnetic field. He named them "X-rays".

Henry Becquerel

Apr 19, 1896

While studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very penetrating rays.

Joseph John Thompson

Apr 20, 1897

Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram. Studied "canal rays" and found they were associated with the proton H + .

Ernest Rutherford

Apr 19, 1898

Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil . He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

Marie & Pierre Curie

Apr 20, 1898

Studied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium.

Soddy

Apr 20, 1900

Observed spontaneous disintegration of radioactive elements into variants he called "isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released during decay.

Nagaoka

Apr 20, 1903

Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.

Abegg

Apr 20, 1904

Discovered that inert gases had a stable electron configuration which lead to their chemical inactivity.

Albert Einstein

Apr 20, 1905

Published the famous equation E=mc2

Hans Geiger

Apr 20, 1906

Developed an electrical device to "click" when hit with alpha particles.

Robert Millikan

Apr 19, 1909

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

Niels Bohr

Apr 19, 1922

Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.

Louis deBroglie

Apr 19, 1923

Discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein.

Erwin Schrödinger

Apr 19, 1930

Viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.

James Chadwick

Apr 19, 1932

James Chadwick discovered the neutron in 1932.

Werner Heisenberg

Apr 19, 1958

One of his most memorable discoveries is the Uncertainty Principle. He said this means that electrons do not travel in neat orbits. Also, all electrons that contain photons will then change momentum and physics.

Max Planck

Apr 19, 2015

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

Aristotle

Mar 2, 2022

He proposed another theory that all matter consisted of four elements: earth, air, water, and fire. He also said that all matter consisted of four qualities: dryness, hotness, coldness, and wetness. (384 BC)