The History of the Atom

Democritus (460 - 370 BC)

460

Democritus first used the term 'atmous' to describe the atom, it meant 'not to be cut'. Democritus' atomic theory stated that atoms were indestructible, they were solid but invisible and that they differ from each other in terms of size, shape, mass, position and arrangement. For example, he believed that solids were composed of small, pointed atoms. His first atomic model consisted of a sphere.

John Dalton (1766 - 1844)

Sep 17, 1766

Dalton's atomic theory stated that all matter is made from atoms, and that atoms are indivisible and indestructible. He also said that all atoms of a given element are identical in both mass and properties. Thirdly, he found that compounds are formed by a combination of two or more different types of atom and that a chemical reaction is simply a re-arrangement of atoms. The essence of Dalton's theory still remains valid today.

Ernest Rutherford (1871 - 1937)

Sep 17, 1871

Rutherford wanted to test out J.J. Thomson's plum pudding model, so he set up an experiment to test it. He fired very fast alpha particles at a thin piece of gold foil. He generated three conclusions from this experiment. Firstly, he concluded that most of the atom is made up of empty space. Secondly, he found that the nucleus must have a positive charge, and thirdly, he found that the small nucleus of the atom must have a large mass relative to the rest of the atom.

Niels Bohr (1885 - 1962)

Sep 17, 1885

Bohr's contribution to the history of the atom, was through his atomic model. He showed the atom as a small, positively charged nucleus surrounded by orbiting electrons. Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.

J.J. Thomson (1856 - 1940)

Sep 17, 1897

J.J. Thomson discovered the electron in 1897. He used this to show that the atom contained smaller pieces, whereas Dalton had thought that atoms could not be broken down into anything simpler. Thomson suggested that atoms contained far fewer electrons, a number roughly equal to the atomic number. As a result of this, Thomson propsed a plum pudding model with positive and negative charge filling a sphere.

Gerd Binnig (1947 - present) and Heinrich Rohrer (1933 - 2013)

Sep 17, 1981

Gerd Binnig and Heinrich Rohrer invented the Scanning Tunneling Microscope (STM) in 1980. This meant that the first atoms could actually be seen. The microscope provided a 3D profile of the surface of the material, which in turn showed that materials were composed of spherical atoms stacked row on row.

Aristotle (384 - 322 BC)

Mar 2, 2022

Aristotle believed that all the materials on Earth were composed of four elements; earth, fire, water and air instead of atoms. He believed that these four elements were used in small amounts to make up all substances. As many people of the time chose to believe in Artistotle's theory, they rejected Democritus' ideas, which were overlooked by approximately 2,000 years. However, Aristotle's ideas were eventually proven to be incorrect.

Plato (428 - 423 BC)

Mar 2, 2022

Plato believed that atoms of matter could be formed from five solid shapes; the triangle, the square, the pentagon and the hexagon. However, at the time it was commonly believed that Aristotle's theory was true (that atoms were composed of earth, fire, water and air), so Plato applied this misconception to his own theory. Fire was represented by a tetrahedron, air was represented by an octahedron, water by a icosahedron and earth by a cube.