History of the Atomic Theory

Democritus

460 BCE

Lived from 460BC to 370BC.
Lived in the Ancient Greek city of Abdera
Democritus believed in natural law and no being that was a god. He thought that atoms had shape, mass, and motion, but no other qualities, such as colour of flavor. He thought that all atoms had various shapes.
Democritus took a seashell and kept cutting it in half until he could not anymore. He thought that you could still keep cutting in half, that atoms were not geometrically divisible, but they are physically.

John Dalton

1803

September 6, 1766
He lived in manchester
His idea of partial pressure was based on his theory that like atoms in a mixture of gases repel one another, where unalike atoms react indifferently towards each other. He stated that all compounds were composed combinations of these atoms in defined ratios. He also said that chemical reactions result in the rearrangement of the reacting atoms.

J.J. Thompson

1894

Born December 18, 1856
He lived in the UK
In 1894 he researched cathode rays, which are glowing beams of light that follow an electrical discharge in a high-vacuum tube. He found that when the rays entered a slit in a pair of metal cylinders, they measured a large amount of negative charge. This led to the discovery of the electron. Through his research, Thompson concluded that all matter is made up of tiny particles that are much smaller than atoms.

Cathode Ray Tube Experiment

1894

In 1894 he researched cathode rays, which are glowing beams of light that follows an electrical discharge in a high-vacuum tube. He found that when the rays entered a slit in a pair of metal cylinders, they measured a large amount of negative charge. This led to the discovery of the electron. Through his research Thompson concluded that all matter is made up of tiny particles that are much smaller than atoms.

Ernest Rutherford

1899

He was born August 30, 1871
He lived in England and Canada all through his life
He discovered some elements give off positively charged particles that will penetrate almost anything. He aimed a beam of alpha particles at a thin sheet of gold foil. the foil was surrounded with a screen that glowed when alpha particles struck it. Most of the alpha particles passed straight through the gold foil as though it wasn’t there. He concluded that a tiny, dense nucleus was causing the defections.

Robert Millikan

1909

Born March 22, 1868
Lived in the US.
He was responsible for the oil drop experiment and it determined the size of the charge of the electron.
The apparatus that he used had an electrical field that was created between a parallel pair of metal plates. Electrically charged oil drops entered the electrical field and were balanced between two plates.
When the oil fell at a consistent rate the gravitational and electrical forces on it were equal. The charge of a single electron was 1.6 x 10-19 C.

Oil Drop Experiment

1909

The oil drop experiment was performed by Robert Millikan in 1909, and attempted to measure the charge of an electron. The apparatus that he used had an electrical field that was created between a parallel pair of metal plates. Electrically charged oil drops entered the electrical field and were balanced between two plates.
When the oil fell at a consistent rate the gravitational and electrical forces on it were equal. The charge of a single electron was 1.6 x 10-19 C.

Gold Foil Experiment

1911

An experiment conducted in 1911 by Ernest Rutherford, the gold foil experiment resulted in the discovery of the nucleus of an atom. The experiment was done by shooting tiny particles of at a thin sheet of gold. A small percentage of the particles were deflected off of the sheet, while a majority of them passed right through it. Rutherford concluded that a tiny, dense nucleus was causing the abnormalities in the results.

Niels Bohr

1913

October 7, 1885
He lived in Copenhagen, Denmark
Bohr proposed that electrons are arranged in concentric circular orbits around the nucleus of an atom. Bohr's model can be described by these four principles:
Electrons occupy only certain orbits around the nucleus.
Each orbit has an energy associated with it.
Energy is absorbed or emitted when an electron jumps from a higher to lower orbit, or a lower to higher one.
The energy and frequency of light emitted or absorbed can be calculated.

Erwin Schrodinger

1926

Born August 12, 1877
Lived in Vienna, Austria
Erwin Shrodinger used mathematical equations to find the likelihood of an electron being in a certain location. They are part of a wave function and act based on how they are observed, superpositioning when seen. Schrodinger introduced the quantum mechanical version of the atom and explained the behavior of electrons. An experiment was performed now known as Schrodinger's Cat, and is a basic understanding of how electrons behave in the quantum world.

James Chadwick

1932

October 20, 1891
He lived in the UK
James Chadwick bombarded beryllium atoms with alpha particles. He observed an unknown radiation being created, with the same mass as a proton and a neutral charge, which came to be known as a neutron. With the discovery of the neutron, an accurate model of the atom became available to chemists.