The Atomic Theory

Leucippus (460 Century BCE)

460 BCE

Founder of atomism in ancient Greek philosophy. His dates are unknown although it is believed that he lived through the fifth century BCE. He is also believed to be the originator of the theory that states that the universe is made up of two different elements which he named the solid and the void. He was also Democritus’ mentor. His theory was believed to come out in the fifth century BC.

Democritus (460 BCE - 370 BC)

460 BCE

He elaborated the atomic theory his mentor, Leucippus came up with. According to him, atoms were miniscule quantities of matter. Democritus also hypothesized that atoms cannot be destroyed, differ in size, shape, and temperature, are always moving, and are invisible. He also believed that there were an infinite number of atoms.

John Dalton (Sep 6, 1766 - Jul 27, 1844)

1766

Dalton based his view of atomism in his meteorology studies. His atomic theory expressed the following: All matter is made of atoms and atoms are indivisible and indestructible, all atoms of a given element are identical in mass and properties, compounds formed by a combination of two or more different kinds of atoms, and a chemical reaction is a rearrangement of atoms. His theory came up in 1803.

Eugen Goldstein (Sep 5, 1850 - Dec 25, 1930)

1850

In 1886 he discovered canal rays also known as positive rays, there positively charged that are accelerated toward and through a perforated cathode in an evacuated tube. He studied cathode rays as well, in 1886 he demonstrated that these rays could cast sharp shadows and that they were emitted perpendicular to the cathode surface. His worked suggested the presence of the proton, a positively charged particle, later discovered by Ernest Rutherford.

Joseph John Thomson (Dec 18, 1856 - Aug 30, 1940)

1856

In 1897 Thomson discovered the electron. In 1904 he suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. Along with his student, Ernest Rutherford, he initiated the research to estimate the number of electrons in an atom. Thomson's last important experimental program was focused on determining the nature of positively charged particles. His assistant, Francis Aston improved Thomson's instrument and discovered isotopes.

Ernest Rutherford (Aug 30, 1871 - Oct 19, 1937)

1871

He proposed the nuclear atom also known as the planetary model of the atom in 1911. This model described the atom as a tiny, dense, positively charged core called a nucleus, where almost all of the mass is concentrated, around which the light, negative components, called electrons, circulate at some distance.

Niels Bohr (Oct 7, 1885 - Nov 18, 1962)

1885

Bohr combined his mentor's, Rutherford, description of the nucleus and Planck's theory about quanta to explain what happens inside an atom and to develop a picture of atomic structure. The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. He was the first to discover that the number of electrons in the outer orbit determines the properties of an element. He is the author of the liquid droplet theory and the concept of complementarity.

James Chadwick (Oct 20, 1891 - Jul 24, 1974)

1891

In 1932 Chadwick created an unknown radiation by bombarding beryllium atoms with alpha particles. He described this radiation as being made up of particles with a neutral electrical charge and the approximate mass of a proton. This particle was later known as the neutron. With this discovery, a suitable model of the atom became available to chemists.