Atomic Theory

Events

Democritus

460 BC

Greek philosopher remembered mostly for his idea of an atomic theory of the universe. His experiment was taking a seashell and breaking it apart until he had a fine powder. After not being able to break a piece of the powder, he determined he had the atom, the indivisible building block of everything.

Antoine Lavoisier

1743

French chemist who came up with the Law of Conservation of Mass. Conducted experiments transforming elements.

John Dalton

1766

English chemist who developed the law of multiple proportions which talked about how elements combine.

Law of Conservation of Mass

1785

Law by Lavoisier stating in a chemical reaction, matter is neither created nor destroyed. Was discovered while doing chemical reactions, the weight didn’t change.

Dalton’s Atomic Theory

1803

Atomic theory proposed by Dalton. It states that all matter is made of atoms that are indivisible and indestructible, all atoms of a given element are identical in mass and property, compounds are formed by the combination of two or more atoms of different kinds, and a chemical reaction is a rearrangement of atoms.

Dmitri Mendeleev

1834

Russian chemist who created the first periodic table. He arranged his table by properties of the elements.

J.J. Thomson

1856

English physicist who discovered the electron with an experiment that produced cathode rays by introducing gas into a region between charged plates. Thomson tried to prove the rays were electrons.

Ernest Rutherford

1871

New Zealand physicist who conducted the Gold Foil experiment. His experiment allowed him to create the Rutherford model of the atom that introduced the nucleus.

Niels Bohr

1885

Danish physicist who proposed the Bohr model of the atom. Also helped develop the atomic bomb.

Erwin Schrodinger

1887

Austrian physicist who proposed the Quantum Mechanical model of the atom. He used equations to predict the location of electrons in atoms.

Henry Moseley

1887

English physicist who repositioned the periodic table. He arranged it based on atomic weight rather than properties.

Cathode Ray Tube

1897

Tube used by Thomson in the Cathode Ray experiment. It was a vacuum with two electric plates that produced a positive anode and a negative cathode to deflect rays.

Plum Pudding Atomic Model

1904

Model of the atom proposed by JJ Thomson. Proposed that the atom was a positively charged sphere that had negatively charged electrons embedded in it, balancing the total charge.

Gold Foil Experiment

1911

Experiment conducted by Rutherford that proved the existence of a small massive center to atoms. This center became known as the nucleus.

Rutherford Model

1911

Model of the atom proposed by Rutherford. It disputed the plum pudding model of the atom. It introduced the nucleus of the atom which was said to contain the bulk of the atomic mass and have a small volume compared to the rest of the atom.

Robert Millikan

1913

American physicist who measured the charge of electrons. He did this in the oil drop experiment where he dropped oil between electrodes and measured the force of the drops.

Bohr Planetary Model

1913

Model of the atom proposed by Bohr and Rutherford. It proposes that the atom has a small positive nucleus with electrons orbiting the nucleus in a circle, like planets around the sun.

Electron Cloud Model

1925

Model of the atom proposed by Schrodinger. It proposed that rather than electrons moving in orbits like the Bohr model proposed, electrons moved in random places within an electron cloud.

Quantum Mechanical Model

1926

Model of the atom proposed by Schrodinger. It was an improved version of the Bohr model that used equations to predict the likelihood of finding an electron in a certain place.

James Chadwick

1930

English physicist who conducted an experiment bombarding Beryllium with alpha particles. The result was a very energetic stream of radiation. These rays were very penetrating and neutral. The rays knocked protons loose and allowed for the neutron particle to be seen.