ATOMIC TIMELINE

DEMOCRITUS

• Atoms are small, hard particles
• Atoms constantly move
• Atoms are "uncuttable"
• All substances are made of atoms
• Found in 440 BC

DALTON

• Atoms of the same element are exactly alike
• Atoms of different elements are different
• Elements combine in specific proportions
• He conducted experiments in combining elements
• Found in 1803

THOMSON

• He conducted the cathode-ray tube experiment
• His theory of atomic structure led to the "plum-pudding" model
• There are small, negatively charged particles inside an atom
• Found in 1897

RUTHERFORD-1911

• There is a small dense, positively charged nucleus
• He conducted the gold foil experiment
• Electrons move in empty space in the atom
• Most of an atoms mass is in the nucleus
• Atoms contain mostly empty space

BOHR

• Electrons travel in definite paths
• Electrons jump between levels from path to path
• Found 1913

MODERN THEORY

• Electron paths cannot be predicted
• Are found in electron clouds, not paths
• Found in 20th century