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Models of the Atom Model Characteristics Greeks - 450 BC Democritus and Leucippus * The word "atom" comes from the Greek “a-tomos" and signifies indivisible. + solid (no void inside) . * surrounded by an empty : Dalton - 1803 John Dalton's Atomic Theory Positively charged matter Electron + All matter consists of atoms, tiny indivisible particles of an element that cannot be created or destroyed. * Atoms of one element CANNOT be converted into atoms of another element. * Compounds form from the chemical combination of a specific ratio of atoms of different elements Scientist's noticed that: . * Cathode rays traveled in straight lines unless they were placed in a magnetic or electrical field +’ When placed in a magnetic field, the cathode rays bent | + When placed in an electrical field, the cathode rays bent towards the positive plate * The cathode ray was identical regardless of the metal used as the cathode oS Id. Thompson 5 teatize Gs at this was evidence that the atom was divisible and he proposed his model of the atom “the raisin bun in a plum ‘pudding” with electrons embedded in a positively charged sphere. Rutherford - 1911 «Tiny positively charged. alpha particles from radioactive radium were aimed like projectiles at a thin gold foil * lin 20,000 alpha particles was deflected back by more than 90° * Something small in the centre of the atom was repelling the alpha particles : + Rutherford concluded that the atom was mostly empty space occupied by electrons but in the centre, a small “nucleus" contained all of the positive charge and essentially all the mass of the atom + Rutherford explained the nature of the atom but not the mass. In 1931 James Chadwick discovered the neutron which explained the mass Bohr - 1913 Bohr hypothesized ‘that hydrogen only had certain allowable energy | s on which electrons could be found 
