Atomic Theory

Democritus

400 BCE

He stated that the universe is made of tiny, invisible atoms that can't be destroyed and that differ in size, shape, and temperature.

Issac Newton

Jan 25, 1704

Newton understood that atoms move and 'swim' around.

John Dalton

Apr 18, 1803

He came up with Dalton's theory.
1. Matter is made up of extremely small atoms.
2. Atoms are indivisible and indestructible.
3. Atoms of an element are identical in size and chemical props.
4. Atoms are different depending on the element.
5. In Chemical Reactions, atoms separate then combine and/or rearrange.

Dimitri Mendeleev

May 27, 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

J.J. Thomason

Dec 1, 1897

Discovered the electron by experimenting with the cathode ray which is negatively charged.

Marie Curie

Dec 21, 1898

Marie Curie discovered the radioactivity part of the periodic table.

Hans Geiger

Oct 15, 1908

Hans Geiger was a German nuclear physicist best known for his invention of the Geiger counter, a device used for counting atomic particles, and for his pioneering work in nuclear physics with Ernest Rutherford.

R.A. Millikan

Dec 13, 1909

His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory.

Ernest Rutherford

Oct 13, 1911

Rutherford overturned J.J. Thomason's theory with a Gold Foil experiment and discovered atoms have a small charged nucleus.

H.G. Mosely

Aug 10, 1912

Discovered that the frequency of X-Ray Radiation has a precise mathematical relationship to the element's atomic number.

Niels Bohr

Jun 25, 1922

Bohr studied the structure of atoms, he realized electrons orbit.

Heisenberg

Apr 27, 1927

Described atoms by means of formula connected to the frequencies of spectral lines. Principle of Indeterminancy - you can not know both the position and velocity of a particle

James Chadwick

Apr 30, 1932

James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron

Lise Meitner

Feb 12, 1939

Austrian physicist who worked on radioactivity and nuclear physics. Otto Hahn and Meitner led the small group of scientists who first discovered nuclear fission of uranium when it absorbed an extra neutron; the results were published in early 1939.