Atomic Theory

Democritus

492 BCE

Everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms, which differ in shape and size. Of the mass of atoms, Democritus said, "The more any indivisible exceeds, the heavier it is".

Aristotle

384 BCE

Believed in the four elements of earth, air, water, and fire. Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter.

Antoine Lavoisier

1783

Lavoisier found that mass is conserved in a chemical reaction. The total mass of the products of a chemical reaction is always the same as the total mass of the starting materials consumed in the reaction. His results led to one of the fundamental laws of chemical behavior: the law of conservation of matter, which states that matter is conserved in a chemical reaction.

Joseph Louis Proust

1794

Law of Definite Proportions (Law of Constant Composition) states that chemical compounds always combine in constant proportions.

John Dalton

1803

Atomic Theory states:
-Elements are made of extremely small particles called atoms.
-Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
-Atoms cannot be subdivided, created, or destroyed.
-Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
-In chemical reactions, atoms are combined, separated, or rearranged.

Michael Faraday

1830

An English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, diamagnetism and electrolysis.

Henri Becquerel

1896

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

J. J. Thompson

1897

Thompson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. In addition, he also studied positively charged particles in neon gas.

Marie and Perrie Curie

1898

Both scientists discovered radium and plutonium

Max Planck

1899

Planck is a German physicist who is considered to be the inventor of quantum theory. In 1899, he discovered a new fundamental constant, which is named Planck's constant, and is, for example, used to calculate the energy of a photon.

Marie and Pierre Curie

1903

Marie and Pierre were awarded the Nobel Peace Prize along with Henri Becquerel in Physics for the discovery of radiation and radioactivity

Albert Einstien

1905

Published an analysis in which he devised a mathematical way to predict both atoms and molecules

Robert Millikan

1908

While a professor at the University of Chicago, Millikan worked on an oil-drop experiment in which he measured the charge on a single electron.

Ernest Rutherford

1911

Rutherford proved Thompson's Plum Pudding structure incorrect. He performed the famous Gold Foil experiment, which led to his discovery of the nucleus. His discovery of the nucleus is said to be his greatest scientific achievement.

Marie and Pierre Curie

1911

Marie and Pierre Curie are awarded Nobel Peace Prize for the discovery of polonium and radium

Niels Bohr

1922

Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons.

James Chadwick

1932

Chadwick discovered the neutron and the making of another model with mass and charge in a very small nucleus