Atomic Theory and periodic table

Democrius

440 BCE

Propose the idea of the atom. Invisible particle that all matter is made of

John Dalton

1803

John Dalton proposed "Dalton's Law" describing the relationship between the components in a mixture of gases.

Dmitri Mendeleev

1864

He produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other. His Periodic Table included the 66 known elements organized by atomic weights.

William Ramsay

Period: 1894 to 1898

Discovered Noble gases, helium, neon, argon, krypton and xenon between 1894 and 1898. The elements added a whole new group to the periodic table and provided the keystone to our understanding of the electronic structure of atoms, and the way those electrons bind the atoms together into molecules.

J.J. Thomson

1897

Thomson set out to prove that the cathode rays produced from the cathode were actually a stream of negatively charged particles called electrons.

Ernest Rutherford

1900

Ernest Rutherford discovered the source of radioactivity as decaying atoms

Niels Bohr

1913

Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities.

Henry Moseley

1913

Developed the application of X-ray spectra to study atomic structure.
Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers. He used self-built equipment to prove that every element's identity is uniquely determined by the number of protons it has. His discovery enabled him to predict confidently the existence of four new chemical elements, all of which were found.