Atomic Theory Timeline

Democritus (460BC-370BC)

460

http://www.universetoday.com/60058/democritus-atom/
Democritus was an ancient Greek philsopher who came up with the first atomic theory. He claimed that everything is made up of atoms which cannot be separated or destroyed and space lies between different sizes and shapes of atoms.

Antoine Lavoisier (1743-1794)

Aug 26, 1743

http://scienceworld.wolfram.com/biography/Lavoisier.html
Lavoisier was a French chemist who became the father of modern chemistry by establishing the Law of Conservation of Mass. He burnt phosphorus and sulfur in air, and proved that the two products weighed more than the original weight.

John Dalton(1766-1844)

Sep 6, 1766

http://www.biography.com/people/john-dalton-9265201
Dalton was an English chemist who was the first scientist to determine and explain the behavior of atoms in terms of their measurement of weight. His interest in gases also helped to discover that all matter is made of small particles.

Dalton's Atomic Theory

Mar 21, 1803

Activity-Changing Model of an Atom
John Dalton predicted that atoms resemble billiard balls. This model shows that all the matter in a substance (ie gold) has the same kind of particles regardless of where it is found.

Dmitri Mendeleev (1834-1907)

Feb 8, 1834

http://www.chemistry.co.nz/mendeleev.htm
Mendeleev was a Russian chemist who created the Periodic Table which showed 63 known elements based on their atomic masses. His first Periodic Table displayed the elements in ascending order of atomic weight and grouping them by similarity of properties.

J.J Thomson (1853-1940)

Dec 18, 1853

http://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/atomic-and-nuclear-structure/thomson.aspx
Joseph John (J.J) Thomson was a British physicist who discovered electrons using a Cathode Ray Tube. He predicted that an atom was a sphere filled with positive matter in which the electrons are positioned as electrostatic forces.

Robert Milikan (1868-1953)

Mar 22, 1868

http://www.nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html
Milikan was an American physicist who was able to accurately determine the amount charge carried by an electron. He also proved that this quantity was the same for all electrons and demonstrated the atomic structure of electricity.

Ernest Ruthorford (1871-1937)

Aug 30, 1871

http://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/atomic-and-nuclear-structure/rutherford.aspx
Ruthorford was a British physicist who discovered alpha and beta rays, identified alpha particles as helium nuclei and most importantly, discovered the nuclear structure of the atom. He also created the Bohr-Ruthorford Model which measures the amount of energy it takes an electron to move.

Niels Bohr (1885-1962)

Oct 7, 1885

http://www.nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-facts.html
Bohr was a Danish physicist who propsed a theory for the structure of a hydrogen atom based on the idea that energy is transferred only in certain quantities. His theory also proved why atoms emit light in fixed wavelengths.

Cathode Ray Tube

Mar 20, 1887

http://creating.pbworks.com/w/page/16595575/Thomson and the cathode ray tube
JJ Thomson used the Cathode Ray Tube when he was trying to discover electrons. Gases were concealed inside it and he concluded from the experiment that opposite charges attract.

Erwin Schrödinger (1887-1961)

Aug 12, 1887

http://www.nobelprize.org/nobel_prizes/physics/laureates/1933/schrodinger-facts.html
Schrödinger was an Austrian physicist who was the fisrt person to calculate a wave-equation that accurately represented the energy levels of atoms. After Bohr's theory of the atom that it can absorb and emit radiation, Schrödinger went deeper into that theory to find out that electrons could be seen as both particles and waves.

Henry Moseley (1887-1915)

Nov 23, 1887

http://www.britannica.com/EBchecked/topic/393528/Henry-Gwyn-Jeffreys-Moseley
Moseley was an English physicist who showed that the major properties of an element are determined by the element's atomic number, not by it's weight. He also predicted that there were only 92 basic elements and 14 rare-earth elements.

James Chadwick (1891-1974)

Oct 20, 1891

http://www.nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-facts.html
Chadwick was an English physicist who proved the existence of neutrons. After 1930 when Herbert Becker and Walter Bothe sent alpha particles against beryllium and noticed radiation was being emitted, Chadwick as able to prove that a neutral particle was being emitted.

Plum Pudding Atomic Model

Mar 21, 1897

http://wwwoutreach.phy.cam.ac.uk/camphy/nucleus/nucleus1_1.htm
When JJ Thomson discovered electrons, he also believed that they were balanced by some sort of positive charge and the distribution of charge was unknown. Thomson proposed a 'plum pudding' model which shows positive and negative charges filling a sphere.

Gold Foil Experiment

Mar 20, 1909

http://kids.britannica.com/comptons/art-124956/Physicist-Ernest-Rutherford-established-the-nuclear-theory-of-the-atom
Ernest Ruthorford established the idea that there is a nucleus in the center of atoms by using the Gold Foil experiment. He shot particles at a piece of gold foil, where some of them bounced back showing how dense the nucleus is and why some atoms were deflecting.

Ruthorford Model

Mar 21, 1911

Activity-Changing Model of an Atom
Ernest Ruthorford's model of the atom shows that there is a positively charged center of the atom called the nucleus. After conducting the Gold Foil experiment in 1909, Ruthorford noticed some of the alpha particles bounced back which helped him conclude the idea that there was a dense particle inside an atom which then helped him create this model.

Bohr Planetary Model

Mar 21, 1915

http://csep10.phys.utk.edu/astr162/lect/light/bohr.html
Niels Bohr's model of an atom was called a "planetary model" of an atom since the neutrons and protons occupy a dense central region (the nucleus) as the electrons orbit the nucleus similar to the planets' orbit around the Sun.

Quantum Mechanical Model

Mar 21, 1926

http://www.sussexvt.k12.de.us/science/The%20Atom/Schrödinger.htm
Erwin Schrödinger created an equation that related the distance an electron that could be found from the nucleus to the potential energy of the electron. Since the creation of this equation, we can determine where to find an electron at any point in time.

Electron Cloud

Mar 21, 2014

http://wiki.answers.com/Q/What_is_the_electron_cloud_model?#slide=2
Niels Bohr predicted that electron clouds showed the most likely places in an atom where an electron can be located as it moves around the nucleus. The denser the cloud, the greater the possibility an electron will be located.