ATOMIC THEORY

Democritus

500

Democritus, greek philospher, discovers

John Dalton

Jan 11, 1803

he proposed "atomic theory" with spherical solid atoms based on physical properites of mass,

Dimitri Mendeleev

Oct 12, 1869

Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law

Sir William Crookes

Oct 12, 1879

Discovered cathode rays had the following properties: travel in straight lines from the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by electric fields and magnets to suggest a negative charge; cause pinwheels in their path to spin indicating they have mass.

JJ Thomson

Oct 12, 1897

Studied "canal rays" and found they were associated with the proton H+ he also used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 108

Rutherford

Oct 12, 1898

Studied radiations emitted from uranium and thorium and named them alpha and beta.He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

Neils Bohr

Oct 12, 1922

Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of sucessive orbital shells of electrons.