John Dalton-1803 *Created the atomic theory *He said everything was made out of small, indestructible particles *He called them atoms *He said that they were all like one another *All have the same atomic weight
Crookes-1870 *He created the Crookes' tube *Discovered cathode rays *Found that they travel in straight lines *Rays produce phosphorous and heat *Can strike any materials
J.J. Thomson-1898 *Discovered the electron *Discovered them through an electric discharge experiment *Developed the plum-pudding atom model *Model is a sphere full of positive matter *Electrons are positioned through electrostatic forces
Ernest Rutherford-1909 *Tested a gold-foil experiment *Made a new model of the atom *He discovered the nucleus *Discovered where all the mass of atoms comes from *Said that there was a positively charged ball in the center of the atom
Neils Bohr-1913 *Stated that electrons move around the nucleus *Electrons move in large orbits *He made his own atomic model *He said that atoms absorb and emit radiation *Sometimes electrons jump, absorbing or emitting radiation
The Periodic Table
*Dimitri Mendeleev came up with a way of organizing elements *He put them in order of atomic mass *He put them in to rows and columns, called periods and groups *He left empty spaces for undiscovered elements to be placed *When he made the groups, he organized them by similar properties *Later, Henry Moseley made improvements in the early 20th century *He arranged the elements by atomic number rather than mass *The atomic number is the number of protons in the atom's nucleus. *Each element has 1 more proton than last *This made organizng the table much easier *With Moseley's improvement, it was clear how many elements were still missing.