History of Atomic Theory

Democritus

400 BCE

Democritus was born in 460 BC and died 370 BC. He lived in Abdera, Greece. Democritus had the idea that everything is composed of atoms which are physically indivisible, but not geometrically indivisible. He took a sea shell and broke it in half. He discovered the first atomic theory and came up with the conclusion that matter is made up of tiny atoms.

John Dalton

1803

Dalton was born on September 6, 1766, and died July 27, 1844. He was born in Eaglesfield, UK. Dalton's atomic theory proposed that all matter was composed of atoms that are indivisible and indestructible. During his research, he found out that all atoms of an element are identical, and different elements have different sizes of atoms and mass. He made several meteorological observations in his first published work. In 1803 Dalton revealed the concept of Dalton's Law of Partial Pressures.

J.J. Thomson

1897

J.J. Thomson was born December 18, 1856, and died August 30, 1940. He lived in Manchester, UK. Thomson discovered that atoms do not account for a positive or negative charge. Thomson showed this by using cathode-ray tubes. By this, he shows that all atoms have tiny negatively charged subatomic particles or electrons.

Robert Millikan

1909

Millikan was born on March 22, 1868, and died December 19, 1953. He lived in Morrison Illinois and San Marino, California. Millikan's oil drop experiment helped quantify the charge of an electron. He put oil between two electric plates and determined the charge. The oil came through a small hole in the top plate and was ionized by an x-ray, making them negatively charged.

Ernest Rutherford

1911

Rutherford was born August 30, 1871, and died October 19, 1937, in Brightwater, New Zealand. Rutherford used the "Gold Foil Experiment" to show how alpha radiation deflected through gold tin foil. He found out that atoms are mostly empty space with a tiny, dense, positively-charged nucleus. Also, he found out that light ejects a proton. In 1911 he created the Rutherford Atomic model which he proved older models wrong by saying the central area contained a nucleus in high concentration.

Niels Bohr

1913

Bohr was born on October 7, 1885, and died November 18, 1962, He lived in Copenhagen, Denmark. Bohr made foundational discoveries concerning the atomic structure and quantum theory. He came up with the Bohr model which used precise equations to explain the general structure of the atom. With a 1912 paper by Darwin which he based his research on, he advanced the theory of orbiting atoms which was supported by Darwin’s writings. He created the Bohr model for the atom.

Erwin Schrödinger

1926

Schrödinger was born on August 12, 1887, and died January 4, 1961. He lived in Vienna, Austria. In 1926 Schrödinger took the Bohr atom model one step further; Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. He is referred to as the father of quantum physics. Because of his research, the atomic model is known as the quantum mechanical model of the atom.

James Chadwick

1932

Chadwick was born on October 20, 1891, and died July 24, 1974. He lived in Cambridge, UK during his life. Chadwick discovered the neutron. He used previous claims that the nucleus was made up of protons and electrons which gave calculations that did not match up. He made a homemade apparatus that detected the neutrons which became a new discovery.