The Atomic Theory

Democritus and Leucippus

442 BCE

-Democritus; born in 460- died 370 BC
-Leucippus; born and died in the 5th century BCE
Democritus was a greek philosopher, co-originated the thought with his teacher leucippus they said that all matter is composed of indivisible elements. They also stated that:
-All apparent changes in matter result from changes in the groupings of atoms.
-There are different kinds of atoms that differ in size and shape.
-And that the properties of matter reflect the properties of the atoms that matter contains.

John Dalton-(1766-1844)

1776

Born: September 6th, 1776
Died: July 27th, 1844
John Dalton's Atomic Theory consisted of:
-All matter is made of atoms. Atoms are invisible and indestructible.
-All atoms of the given element are identical in mass and properties.
-Compounds are formed by a combination of two or more different kinds of atoms.
-A chemical reaction is a rearrangement of atoms.
Dalton's importance in the development of modern atomic theory has been recognized by the designation of the atomic mass unit.

Eugen Goldstein (1886)

1886

Born; 5th September, 1850
Died; 25th December, 1930
In 1886, Goldstein noted that the cathode-ray tubes with a perforated cathode emit a glow from the end of the tube near the cathode. He noticed that when the cathode of a cathode-ray tube was perforated, he called them "Canal Rays". And since these canal rays travel in the opposite direction from the cathode rays, they must carry the opposite charge.His discovery led to the discovery of the proton.

Joseph John J.J Thomson (1897-1940)

1897

Born: December 18th, 1856
Died: 30th August, 1940
In 1897, Thomson discovered the electron and then went on the propose a model for the structure of the atom. His work also lead to the invention of the mass spectrograph. He developed the Plum Pudding model before the atomic nucleus was discovered. Today, J.J Thomson's discoveries have helped people to have a better understanding of the atom and its generic makeup.

Ernest Rutherford (1911)

1911

Born: 30th August, 1871
Died:19th October 1937
The Rutherford atomic model, described the atom as a tiny, dense positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light and negative constituents are called electrons. Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Although, he reasoned that Thomson's theory was correct.

Niels Bohr (1912)

1912

Born: October 7th, 1885
Died: November 18th, 1962
In 1912, Bohr was working for the nobel Laureate J.J. Under Rutherford's tutelage, Bohr began studying the properties of atoms.The discoveries of the electron and radioactivity at the end of the 19th century lead to different models for the structure of the atom. He proposed the theory for the Hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities.

James Chadwick (1932)

1932

Born: 20th October, 1891.
Died: July 24th, 1974. Chadwick discovered the neutron using evidence collected by Irene Joliot-Curie. He discovered that this beam was not deflected by either electric or magnetic fields, meaning it contained neutral particles-neutrons. He made a fundamental discovery in the domain on nuclear science.At this time did not believe he had discovered a new elementary particle. He believed the neutron to be a complex particle consisting of a proton and an electron.