Atom Theory Timeline

Events

Democritus

400 BC

First thought of and "invented" atoms. Called them Atomus. Atomus means particle incapable of being divided. atomic theory stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.” To find atoms, he divided up a seashell until he could no longer divide them.

John Dalton

1803

Dalton had 5 principles to his theory. 1.Elements are made of extremely small particles called atoms. 2.Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. 3.Atoms cannot be subdivided, created, or destroyed. 4.Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5.In chemical reactions, atoms are combined, separated, or rearranged.

Robert Millikan

1909

Discovered unit charge of electron using his Oil Drop Experiment. The Oil Drop Experiment is the experiment entailed observing tiny charged droplets of oil between two horizontal metal electrodes.

Ernest Rutherford

1911

Designed the Gold Foil Experiment to show that there was a nucleus in the atom. The experiment used alpha particles emitted by a radioactive element as probes for the atomic world

Niels Bohr

1913

Bohr's greatest contribution to modern physics was the atomic model. He was the first to discover that electrons travel around the nucleus. He also discovered that the number of electrons helps decide what the element is. According to his liquid droplet theory, a liquid drop provides an example of a nucleus.

Werner Heisenberg

1925

Heisenberg's contribution to the atomic theory was that he studied behavior of electrons. He found out the the electrons weren't in an orbit, bouncing around in a cloud instead.

Erwin Schrödinger

1926

In addition to Heisenberg, Schrodinger also figured out that the electrons were in a cloud instead of an orbit. He created the model to prove this theory of his. It consisted of a dense nucleus surrounded by a cloud of electrons at various levels in orbitals.

James Chadwick

1932

In 1932, an unknown radiation was produced due to Chadwick bombarding beryllium atoms with alpha particles. This unknown radiation is a neutron, meaning Chadwick was the first to discover the neutron. Due to this discovery, scientists now had and adequate model of the atom