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Democritus (about 450 BC)

450

Democritus' theory was "everything is physically made op of atoms. He stated that atoms are indivisible and can never be broken down into pieces." His atomic model was just simply made of a sphere that is invisible, with no nucleus or electrons. He stated that that atoms are different in size, shape, and also temperature. He had drawn his model showing that atoms cannot be destroyed in any way or form possible.

Aristotle (about 400 BC)

500

Aristotle’s theory states that "all matter is made of the four elements: fire, water, earth, and air." He also stated that "there were four qualities to these elements: dryness, hotness, coldness, and moistness."

John Dalton (1766-1844)

Oct 1, 1803

Dalton is known as the father of atomic theory. His theory states that "All matter is made of atoms, atoms 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, a chemical reaction is a rearrangement of atoms, and atoms can be neither created nor destroyed."

Henri Becquerel

Dec 15, 1852

Discovered radioactivity by accidentally putting crystal in the same drawer as a piece of photographic paper. He was going to do a different experiment, but it was canceled because of bad weather. When it started to clear up, he went to get his tools. He saw that the piece of material, which contained Uranium, could produce an image on the photographic paper even in the absence of light.

Marie & Pierre Curie

Oct 25, 1867

Helped Henri Becquerel with his discovery of radioactivity between 1898 and 1902. Marie, Pierre, and Henri all thought the Atom looked as though it was a planet with many electron rings around them.

J. J. Thomson (1856-1940)

Oct 21, 1897

Thomson's atomic theory is has multiple names, such as "Plum Pudding" and "Chocolate Chip Cookie" model. Thomson had first used the Cathode Ray Tube experiment, in which he had found a negative particle, called electrone. He had then done the chocolate chip cookie model. His atomic theory stated that the atom is not indivisible as it was of smaller pieces, protons and electrons. It also stated that "an atom consists of a sphere of positive charge with negatively charged electron embedded in it."

Robert Millikan

Oct 23, 1908

Millikan discovered the charge of an electron with his oil drop experiment, which is the mass of a hydrogen.

Ernest Rutherford

Oct 23, 1910

The electron cloud model is an atomic model where in electrons are no longer depicted as particles moving around the nucleus in a fixed orbit. Rutherford found that a narrow beam of alpha-particles was broadened when it passed through a thin film of metal. Rutherford discovered protons positively charged particles using the gold foil experiment he determined that atoms were made up of mostly empty space the which the electron moved, and the nucleus that holds all the positively charged particles

Niels Bohr

Oct 25, 1915

Assumed that electrons travel in stationary orbits defined by their angular momentum. He thought that electrons orbit the nucleus in “shells”. Worked on the U.S. Manhattan Project undercover as Nicholas Baker. This project was to develop first atomic bombs during World War II.

James Chadwick

Oct 23, 1932

Made a fundamental discovery: he proved the existence of neutrons. It solved the "jigsaw puzzle" of the atom, and earned the Nobel Prize for Physics in 1935.

Quantum Model

Oct 25, 2012

Explained the stability of Bohr's model. It is what we use today for the atomic models.